Anybody want to discuss this?
- Thread starter royinidaho
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James Jones
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Roy , I read an article somwhere that discusses this same idea , even had some of the same photos. I'm sure that on paper and on the cumputer screen that its a great setup but in reality I don't think that it will make such a huge differance. it has alot to do with than just shoulder angle and shape or the Weatherby line would be more popular., it has to do with the case lenght to the bullet base, that why so far only small caliber have been worked with , it would take like a 50BMG case or somthing to make this work with a 30 cal bullet.
Ralph Payne made a round back when that was sort of the same idea , it was called the 7mm Venturan , it was a short Mag cas with a radiused shoulder , supposidly got close to 3300fps with 160 class bullets
I also had a friend that did this kind of round , it was a 378Wby case shortened to 2.25" and necked to 7mm , he got to 3300fps with 160 class bullets and 80 odd grains of 4831 , barrel life was short and the velocity was not what he expected. He got his idea from the 30-378 Arch which is the same case but has a standard shoulder around 30 degs instead fo the radiused one that my buddy used.
Its hard to get ones head around but their is a reason that some of the same case desgines have been around for so long , for as long as bullets have been stuffed into brass cases guys have been trying to get more out of their guns , guys like Parker O. Ackley , Dr. E.L. Arch , Fred wade , Ralph Payne and our very own Kirby Allen have streched , pushed formed stuffed and worked trying to get the very most out of a long range gun
Ralph Payne made a round back when that was sort of the same idea , it was called the 7mm Venturan , it was a short Mag cas with a radiused shoulder , supposidly got close to 3300fps with 160 class bullets
I also had a friend that did this kind of round , it was a 378Wby case shortened to 2.25" and necked to 7mm , he got to 3300fps with 160 class bullets and 80 odd grains of 4831 , barrel life was short and the velocity was not what he expected. He got his idea from the 30-378 Arch which is the same case but has a standard shoulder around 30 degs instead fo the radiused one that my buddy used.
Its hard to get ones head around but their is a reason that some of the same case desgines have been around for so long , for as long as bullets have been stuffed into brass cases guys have been trying to get more out of their guns , guys like Parker O. Ackley , Dr. E.L. Arch , Fred wade , Ralph Payne and our very own Kirby Allen have streched , pushed formed stuffed and worked trying to get the very most out of a long range gun
Buffalobob
Well-Known Member
Somewhere in my computer a few month I wrote up a long discussion on this and then decided not to post it because it was so negative.
JD hits the same points in only a few sentences. Being as Alabama lost in the second round I guess I have some time. The Weatherby double radius is simply a venturi orifice. It has less pressure loss than the regular angled shoulder cartridges if the distance from the shoulder to the neck is the same. Burning powder and gas will flow smoother through the curved constriction than the sharp angled constriction. You can check any hydraulic pipe fitting manual on pressure headloss. Check thehead loss of flow meters in a irrigation system. Same thing goes for air flow if you should happen to have any mechanical engineering references around. Out in the field you will see Parchal and Palmer Bowles and verturi flumes for measuring irrigation water although simple wiers are the most common. In any event it does not matter what kind of flow you are measureing the curved "appraoch" to the constriction is best for reducing headloss.
I am sure the reason the the case in question has only one shoulder curve is because Weatherby has patented the double curve. A case where half a$$ed is only half as good as full a$$ed. A little funny joke there being as I like Wbys.
So what has the guy invented - a runty half a$$ed Wby.
Now then what does it all mean. Not much is my opinion. I think the guy was bored and decided that here was something that nobody else had done and he did it. Now all that is left is for somebody else to take a case and have the first approach constriction angled and the junction with the neck radiussed. The minor reduction of inside case pressure is not really going to reduce case stretching very much and all this about sparks flying around inside the case ignores the basic principles of the physical universe. Two pieces of matter cannot occupy the same space at the same time and you cannot make sparks and presusre waves fly through a case that is full of powder and focus in the neck.
If this response insults anybody I will be happy to delete it. I try not to post very many "negative remarks" and that is why I never posted my thought before.
JD hits the same points in only a few sentences. Being as Alabama lost in the second round I guess I have some time. The Weatherby double radius is simply a venturi orifice. It has less pressure loss than the regular angled shoulder cartridges if the distance from the shoulder to the neck is the same. Burning powder and gas will flow smoother through the curved constriction than the sharp angled constriction. You can check any hydraulic pipe fitting manual on pressure headloss. Check thehead loss of flow meters in a irrigation system. Same thing goes for air flow if you should happen to have any mechanical engineering references around. Out in the field you will see Parchal and Palmer Bowles and verturi flumes for measuring irrigation water although simple wiers are the most common. In any event it does not matter what kind of flow you are measureing the curved "appraoch" to the constriction is best for reducing headloss.
I am sure the reason the the case in question has only one shoulder curve is because Weatherby has patented the double curve. A case where half a$$ed is only half as good as full a$$ed. A little funny joke there being as I like Wbys.
So what has the guy invented - a runty half a$$ed Wby.
Now then what does it all mean. Not much is my opinion. I think the guy was bored and decided that here was something that nobody else had done and he did it. Now all that is left is for somebody else to take a case and have the first approach constriction angled and the junction with the neck radiussed. The minor reduction of inside case pressure is not really going to reduce case stretching very much and all this about sparks flying around inside the case ignores the basic principles of the physical universe. Two pieces of matter cannot occupy the same space at the same time and you cannot make sparks and presusre waves fly through a case that is full of powder and focus in the neck.
If this response insults anybody I will be happy to delete it. I try not to post very many "negative remarks" and that is why I never posted my thought before.
It's been quite a while since physics class, but I'm thinking the powder wouldn't allow a "shockwave" to occur as in the drawings. The burning powder should contain the wave and disperse the effects. I would think hydraulic action of the shoulder would have greater influence on the burn than the "shock wave" would. just my opinion.
royinidaho
Well-Known Member
Meister,
You are hitting around where I have interest.
In the image linked, I don't understand how what is being shown is being explained. The powder can't begin burning at the base of the bullet can it?
I think that buffalobob expressed the thought that everything that happened in the chamber was a chemical reaction. I look at the moment of initial ignition as combination of chemical (if that's what a controlled explosion is) and mechanical reactions.
Igintion happens, powder burning begins at or near the flash hole followed by mechanical movement of unburned powder and bullet into the bore, if you haven't crimped the bullet as with the hornet. Then powder continues to burn beyond the chamber as the bullet progresses toward the muzzle.
Shoulder angle/curve seems to be designed to deflect the expanding gas and moving powder onto the neck of the case rather than onto the throat.
Am I correct in believing that all the powder does not burn in the case? If not, why not? I'm thinkingto Kirby's powder bridging discussion.
Also do the gases flow, expand or both to make push the bullet? It seems that expansion it the prime mover as I can see the muzzle flash which appears as a flow of gas, which doesn't seem to have much velocity.
My only studies/experience has been with steam turbine nozzles. All rifle cartridges are convergent nozzles which have an upper limit of rate of gaseous flow. Weatherby's idea, hasn't their patent expired, seems a new twist but its still convergent.
To gain any addtional gaseous velocity advantage one would have to design a convergent-divergent nozzle, pretty much like the space shuttle main booster engine has. It would be easy to design but would be one helluva task to rig up an action to handle it. Talk about case extraction problems. /ubbthreads/images/graemlins/grin.gif /ubbthreads/images/graemlins/grin.gif
You are hitting around where I have interest.
In the image linked, I don't understand how what is being shown is being explained. The powder can't begin burning at the base of the bullet can it?
I think that buffalobob expressed the thought that everything that happened in the chamber was a chemical reaction. I look at the moment of initial ignition as combination of chemical (if that's what a controlled explosion is) and mechanical reactions.
Igintion happens, powder burning begins at or near the flash hole followed by mechanical movement of unburned powder and bullet into the bore, if you haven't crimped the bullet as with the hornet. Then powder continues to burn beyond the chamber as the bullet progresses toward the muzzle.
Shoulder angle/curve seems to be designed to deflect the expanding gas and moving powder onto the neck of the case rather than onto the throat.
Am I correct in believing that all the powder does not burn in the case? If not, why not? I'm thinkingto Kirby's powder bridging discussion.
Also do the gases flow, expand or both to make push the bullet? It seems that expansion it the prime mover as I can see the muzzle flash which appears as a flow of gas, which doesn't seem to have much velocity.
My only studies/experience has been with steam turbine nozzles. All rifle cartridges are convergent nozzles which have an upper limit of rate of gaseous flow. Weatherby's idea, hasn't their patent expired, seems a new twist but its still convergent.
To gain any addtional gaseous velocity advantage one would have to design a convergent-divergent nozzle, pretty much like the space shuttle main booster engine has. It would be easy to design but would be one helluva task to rig up an action to handle it. Talk about case extraction problems. /ubbthreads/images/graemlins/grin.gif /ubbthreads/images/graemlins/grin.gif
Buffalobob
Well-Known Member
[ QUOTE ]
I think that buffalobob expressed the thought that everything that happened in the chamber was a chemical reaction.
[/ QUOTE ]
My thought is that you are pushing a "fluidized bed" of hot gas and granular pellets through a constriction we call a "neck". For a given approach length (shoulder to neck distance) a venturi is better than a hard angle and this can be proven from fluid mechanics. Powder bridging is more likely to occur with the angle than the radius. And more likely to occur with a short "appraoch" and a high ratio of constriction (large diameter body, 40 Degree shoulder and narrow neck- extreme example would be a 40 degree shoulder on a 50 BMG necked down to 17 caliber). The job of the approach to a constricted area is to try to acheive laminar flow and reduce turbulence if you want to get the least amount of pressure inside the case and get as much of the burning to occur in the barrel as possible.
[ QUOTE ]
Also do the gases flow, expand or both to make push the bullet?
[/ QUOTE ]
There is gas in the voids of the powder granules. This gas is just regular air but a gas nonetheless. The primer spits out some fire which is a solid that is being converted to gas via exothermic reaction (burning). This causes there to be more gas in the case and the hot gas ignites powder at the base of the case and for a distance through the case. This powder ignition also creates gas. Pressure increases uniformly in the case in all directions even though there are powder granules in the case and at some point the pressure exceeds the neck tension on the bullet and it begins to move down the barrel and gas and granules "flow". The resistance to flow by the friction of the case wall and approach to the neck is determined by several things, one of which is the shape of the shoulder area. The other principle controlling factor is the length of the approach and it is greatly more important than the shape. Sharp shoulders are short shoulders and short vs long is maybe 5-10 times more important than curved versus angled. Long curved is (from a fluid flow standpoint) greatly better than short angled. The reason people like Ackly-ed cases is that the sharp short shoulder resists flow and the gas impacting on it drives shoulder into the chamber shoulder wall creating a flatter pont of resistance to body growth. So we see flow resistance can be our friend for some purposes and our enemy for others
[ QUOTE ]
All rifle cartridges are convergent nozzles which have an upper limit of rate of gaseous flow.
[/ QUOTE ]
That is correct (except for the straight walled cases) and what I am saying is that the curved approach causes the upper limit of flow to be higher than an angled approach.
[ QUOTE ]
convergent-divergent nozzle
[/ QUOTE ]
You do not want a convergent divergent neck. Convergence build up pressure from trying to squeeze through the constriction. You want the least amount of convergence possible and you want it as smooth as possible if you are trying to gain maximum veloicty. The 22 hornet case is the best fluid flow case I know of because it has a very very long shallow approach to the neck. A straight walled case such as a 45-70 has zero convergence and is from a fluid flow concept the very best, but it has a limited powder capacity.
Think about a 50 BMG with a constriction of 17 REm and then back to 50 caliber. You would have so much friction loss trying to get the hot gas and granules through the cosntriction that I would guess you would lose more than 25% of your barrel pressure and velocity. The reason a rocket ship has a constriction is to build up extreme pressures against the rocket ship not against the outside world which may be a vacuum. The cone just gets the hot burning rocket fuel away from the base of the rocket and helps with turbulence and directional stabilty.
My thoughts on the case in question all just revolve around fluid flow. There are other considerations in the world.
I think that buffalobob expressed the thought that everything that happened in the chamber was a chemical reaction.
[/ QUOTE ]
My thought is that you are pushing a "fluidized bed" of hot gas and granular pellets through a constriction we call a "neck". For a given approach length (shoulder to neck distance) a venturi is better than a hard angle and this can be proven from fluid mechanics. Powder bridging is more likely to occur with the angle than the radius. And more likely to occur with a short "appraoch" and a high ratio of constriction (large diameter body, 40 Degree shoulder and narrow neck- extreme example would be a 40 degree shoulder on a 50 BMG necked down to 17 caliber). The job of the approach to a constricted area is to try to acheive laminar flow and reduce turbulence if you want to get the least amount of pressure inside the case and get as much of the burning to occur in the barrel as possible.
[ QUOTE ]
Also do the gases flow, expand or both to make push the bullet?
[/ QUOTE ]
There is gas in the voids of the powder granules. This gas is just regular air but a gas nonetheless. The primer spits out some fire which is a solid that is being converted to gas via exothermic reaction (burning). This causes there to be more gas in the case and the hot gas ignites powder at the base of the case and for a distance through the case. This powder ignition also creates gas. Pressure increases uniformly in the case in all directions even though there are powder granules in the case and at some point the pressure exceeds the neck tension on the bullet and it begins to move down the barrel and gas and granules "flow". The resistance to flow by the friction of the case wall and approach to the neck is determined by several things, one of which is the shape of the shoulder area. The other principle controlling factor is the length of the approach and it is greatly more important than the shape. Sharp shoulders are short shoulders and short vs long is maybe 5-10 times more important than curved versus angled. Long curved is (from a fluid flow standpoint) greatly better than short angled. The reason people like Ackly-ed cases is that the sharp short shoulder resists flow and the gas impacting on it drives shoulder into the chamber shoulder wall creating a flatter pont of resistance to body growth. So we see flow resistance can be our friend for some purposes and our enemy for others
[ QUOTE ]
All rifle cartridges are convergent nozzles which have an upper limit of rate of gaseous flow.
[/ QUOTE ]
That is correct (except for the straight walled cases) and what I am saying is that the curved approach causes the upper limit of flow to be higher than an angled approach.
[ QUOTE ]
convergent-divergent nozzle
[/ QUOTE ]
You do not want a convergent divergent neck. Convergence build up pressure from trying to squeeze through the constriction. You want the least amount of convergence possible and you want it as smooth as possible if you are trying to gain maximum veloicty. The 22 hornet case is the best fluid flow case I know of because it has a very very long shallow approach to the neck. A straight walled case such as a 45-70 has zero convergence and is from a fluid flow concept the very best, but it has a limited powder capacity.
Think about a 50 BMG with a constriction of 17 REm and then back to 50 caliber. You would have so much friction loss trying to get the hot gas and granules through the cosntriction that I would guess you would lose more than 25% of your barrel pressure and velocity. The reason a rocket ship has a constriction is to build up extreme pressures against the rocket ship not against the outside world which may be a vacuum. The cone just gets the hot burning rocket fuel away from the base of the rocket and helps with turbulence and directional stabilty.
My thoughts on the case in question all just revolve around fluid flow. There are other considerations in the world.
royinidaho
Well-Known Member
Roger that!
Fiftydriver
Official LRH Sponsor
Only had time to look at the first page of graphic. This is ML McPersons design is it not. Personally, I think the shock wave vectors are greatly exaggerated to make his design look much better then it really is in real life.
This is all fine and dandy but once the bullet starts to move what difference does it make.
You can do the exact same thing with forward ignition and get even better results. More time involved but much better results.
I do not see the advantage in this design to be honest. But any experimenting is interesting reading!!
Kirby Allen(50)
This is all fine and dandy but once the bullet starts to move what difference does it make.
You can do the exact same thing with forward ignition and get even better results. More time involved but much better results.
I do not see the advantage in this design to be honest. But any experimenting is interesting reading!!
Kirby Allen(50)
I hate to Phooey on a guys parade, but I'm with KA on this one. His idea would hold weight in an empty case, but that would be useless. His idea is saying his shoulder design makes the frontal ignition possible. The primer charge bypasses all the powder at the base of the case and because of the increased pressure behind the bullet, it burns faster? Na, not buyin.
The thought of bridging interests me. Would it be a greater difficulty in small throat cases such as 22-264 cal? I always wondered about the internal dynamics of a large bodied, small neck case with regard to extruded powder.
The thought of bridging interests me. Would it be a greater difficulty in small throat cases such as 22-264 cal? I always wondered about the internal dynamics of a large bodied, small neck case with regard to extruded powder.
goodgrouper
Well-Known Member
I don't know if the Weatherby shoulder is all it is cracked up to be, but the sharper shoulder angle does in my experience do what it's inventor intended it to do. I am of course talking about P.O. Ackley. He realized that a 35 or 40 degree shoulder would indeed get more velocities from slower burning powders such as H4831 that were abundant in large quantities after WWII. A few other guys later found that it had even more benefits when using heavy-for-caliber bullets.
I have had many 40 degree shouldered cartridges and have witnessed many others get unreal ballistics when compared to other cases with similiar case capacities but shallower shoulder angles.
Take my 338 thunder for example. My barrel is 4" shorter than most of the other guys 338 edges and has less case capacity, yet I am getting roughly the same velocities with less powder. Why? Well, all other things being equal (which I admit is hard to do), the fast velocities I'm getting could be a direct result of the sharp shoulder and slow powder combination. Now there is not much difference between the 30 degree shoulder of the Edge and the 40 degree shoulder of the Thunder but it must be enough. It may not be as significant as the difference between the 220 swift's 21 degree shoulder and the 22-250 AI's 40 degree shoulder, but it is still there.
To this day, I have yet to see a fast twist 220 swift shoot an 80 grain bullet as fast as a 22-250 AI will shoot it. Case capacity on these two cartridges is so similiar that when a good dose of a slow powder like RL22 or H4831 is used, they use the same charge weight to get top pressures. But the 22-250Ai will often be 100-200 fps faster!
Just my experiences here and they might not be worth a sack of potatos! /ubbthreads/images/graemlins/wink.gif
I have had many 40 degree shouldered cartridges and have witnessed many others get unreal ballistics when compared to other cases with similiar case capacities but shallower shoulder angles.
Take my 338 thunder for example. My barrel is 4" shorter than most of the other guys 338 edges and has less case capacity, yet I am getting roughly the same velocities with less powder. Why? Well, all other things being equal (which I admit is hard to do), the fast velocities I'm getting could be a direct result of the sharp shoulder and slow powder combination. Now there is not much difference between the 30 degree shoulder of the Edge and the 40 degree shoulder of the Thunder but it must be enough. It may not be as significant as the difference between the 220 swift's 21 degree shoulder and the 22-250 AI's 40 degree shoulder, but it is still there.
To this day, I have yet to see a fast twist 220 swift shoot an 80 grain bullet as fast as a 22-250 AI will shoot it. Case capacity on these two cartridges is so similiar that when a good dose of a slow powder like RL22 or H4831 is used, they use the same charge weight to get top pressures. But the 22-250Ai will often be 100-200 fps faster!
Just my experiences here and they might not be worth a sack of potatos! /ubbthreads/images/graemlins/wink.gif
Fiftydriver
Official LRH Sponsor
Meister,
Powder bridging will result from several factors, the main ones being neck diameter, shoulder angle, shoulder diameter, powder charge size and powder granular size.
My personal experience with powder bridging has been with my 257 AM and 6.5mm AM rounds. With these case designs we are dealing with a 92 to 95 gr powder charge depending on powder used, a shoulder that is as wide as you can get using a RUM sized case and a shoulder angle that is not quite 40 degrees but close.
Combine all those with a neck diameter or roughly .257" and .264" and stick powders tend to get real friendly and lock up tight when fired in these cases. In fact, every stick powder I have every tested has resulted in this porblem. Even the relatively short RL powders.
Jumping up to the 270 AM you have an increase in neck diameter but also in case capacity but this does not seem to result in as much of a problem with powder bridging even though the shoulder diameter and shoulder angle is the same as the smaller caliber AMs.
It seems once the neck diameter gets to 270 and larger, powder bridging really drops off and becomes a none problem in most cases, at least with cases up to that of the RUM class of capacity.
I have not tested my 277 AM with stick powders to see if this happens.
I have also seen this, but to a much more limited amount in the 257 STW. Only a couple rifles I have ever built in this chambering have shown a likelyhood for this problem. Most never do and I am not sure why the few that did, did!!
May be variables in differnet lots of powder that cause this.
Far and away, shoulder angle, shoulder diameter, neck diameter are the three MAIN reasons for this occuring. Powder charge and powder size are high on the list but secondary factors from what I have seen.
Would a 22-264 have powder bridging issues, probably with stock powders I would expect it.
As to the benefits of a sharp shoulder case design over a shallower shoulder angle. Certainly these sharp shoulders restrain the powder in the case longer then a shallow shoulder design will.
I also feel they promote powder ignition better as well to some degree. They also control case stretching better but proper case sizing is the biggest factor to this with any case design.
I do not feel they are any more accurate then a shallow shouldered design.
As far as higher velocity potential, there are alot of other factors that can come into play here as well, throat design, neck fit, case body fit, bore tightness, powder lot variations.
I would say with most wildcats, my Allen Magnums included, they generate more velocity then traditional rounds for the most part because the chambers are designed to handle higher pressure levels because they are designed extremely tight speced to the cases that will be fired in them.
This allow them to opperate at chamber pressures much higher then what is common for a standard spec chamber.
One dramatic example of this is my 270 Allen Mag. When I initially designed the reamer I used min specs on the case body diameter from SAAMI charts for the 7mm RUM. Only problem is that the actual cases were generally much smaller then this. When I say much smaller I am talking about 6 to 7 thou smaller in diameter.
When running up to FULL tilt pressures, my original 270 AM rifle will top out at around 3300 fps at which point the primer pockets will begin to loosen.
I redesigned the chamber and tightened up the case body diameter in relation to the actual cases that would be fired in the rifle. Now the chamber is only a 2.5 thou larger in diameter then the actual case. Thats only 1.25 thou over case capacity. Not much room for expansion.
As a result, the cases fired in the newly designed chamber would produce well over 3400 fps and in some cases push 3450 and even 3500 fps as reported by Bill Bailey using H-US869. All these with the 169.5 gr ULD RBBT.
SO why the increase in velocity, every other dimension was identical and it simply results from the chamber being designed to handle more pressure and still controlling case head expansion better.
There is also something to the shorter fatter case theory but personally I believe this has much more to do with usible barrel length then it does the actual design of the case.
You put the same volume of powder under the same bullet but with one bullet having 25" of usible barrel life and another having another inch of usible barrel life. Even though they both have the same capacity, the shorter case will produce more velocity, everytime. Just a matter of barrel time for the bullet. The longer its in a bore under pressure, the faster it will go, so to speak.
Interesting topic!!
Kirby Allen(50)
Powder bridging will result from several factors, the main ones being neck diameter, shoulder angle, shoulder diameter, powder charge size and powder granular size.
My personal experience with powder bridging has been with my 257 AM and 6.5mm AM rounds. With these case designs we are dealing with a 92 to 95 gr powder charge depending on powder used, a shoulder that is as wide as you can get using a RUM sized case and a shoulder angle that is not quite 40 degrees but close.
Combine all those with a neck diameter or roughly .257" and .264" and stick powders tend to get real friendly and lock up tight when fired in these cases. In fact, every stick powder I have every tested has resulted in this porblem. Even the relatively short RL powders.
Jumping up to the 270 AM you have an increase in neck diameter but also in case capacity but this does not seem to result in as much of a problem with powder bridging even though the shoulder diameter and shoulder angle is the same as the smaller caliber AMs.
It seems once the neck diameter gets to 270 and larger, powder bridging really drops off and becomes a none problem in most cases, at least with cases up to that of the RUM class of capacity.
I have not tested my 277 AM with stick powders to see if this happens.
I have also seen this, but to a much more limited amount in the 257 STW. Only a couple rifles I have ever built in this chambering have shown a likelyhood for this problem. Most never do and I am not sure why the few that did, did!!
May be variables in differnet lots of powder that cause this.
Far and away, shoulder angle, shoulder diameter, neck diameter are the three MAIN reasons for this occuring. Powder charge and powder size are high on the list but secondary factors from what I have seen.
Would a 22-264 have powder bridging issues, probably with stock powders I would expect it.
As to the benefits of a sharp shoulder case design over a shallower shoulder angle. Certainly these sharp shoulders restrain the powder in the case longer then a shallow shoulder design will.
I also feel they promote powder ignition better as well to some degree. They also control case stretching better but proper case sizing is the biggest factor to this with any case design.
I do not feel they are any more accurate then a shallow shouldered design.
As far as higher velocity potential, there are alot of other factors that can come into play here as well, throat design, neck fit, case body fit, bore tightness, powder lot variations.
I would say with most wildcats, my Allen Magnums included, they generate more velocity then traditional rounds for the most part because the chambers are designed to handle higher pressure levels because they are designed extremely tight speced to the cases that will be fired in them.
This allow them to opperate at chamber pressures much higher then what is common for a standard spec chamber.
One dramatic example of this is my 270 Allen Mag. When I initially designed the reamer I used min specs on the case body diameter from SAAMI charts for the 7mm RUM. Only problem is that the actual cases were generally much smaller then this. When I say much smaller I am talking about 6 to 7 thou smaller in diameter.
When running up to FULL tilt pressures, my original 270 AM rifle will top out at around 3300 fps at which point the primer pockets will begin to loosen.
I redesigned the chamber and tightened up the case body diameter in relation to the actual cases that would be fired in the rifle. Now the chamber is only a 2.5 thou larger in diameter then the actual case. Thats only 1.25 thou over case capacity. Not much room for expansion.
As a result, the cases fired in the newly designed chamber would produce well over 3400 fps and in some cases push 3450 and even 3500 fps as reported by Bill Bailey using H-US869. All these with the 169.5 gr ULD RBBT.
SO why the increase in velocity, every other dimension was identical and it simply results from the chamber being designed to handle more pressure and still controlling case head expansion better.
There is also something to the shorter fatter case theory but personally I believe this has much more to do with usible barrel length then it does the actual design of the case.
You put the same volume of powder under the same bullet but with one bullet having 25" of usible barrel life and another having another inch of usible barrel life. Even though they both have the same capacity, the shorter case will produce more velocity, everytime. Just a matter of barrel time for the bullet. The longer its in a bore under pressure, the faster it will go, so to speak.
Interesting topic!!
Kirby Allen(50)
royinidaho
Well-Known Member
Meister,
I think Kirby mentioned the pressure spike that results from the bridging.
BTW, thanks for the response on long range optics.
I think Kirby mentioned the pressure spike that results from the bridging.
BTW, thanks for the response on long range optics.
That's why I'm here, Learn what I can and Tell what I have learned. This is the best board on the web, low bs, high content value. I'd like to keep it that way, len's living room. I prefer the back porch myself... Generally closer to the kitchen, and the beer!
