Well, I am not sure what rounds you are referring to that produce 50 to 65K in pressure before the bullet begins to move but this simply is not how things happen inside a rifle when a round it touched off.
The bullet will start moving under only a few thousand pounds of pressure if it is not seated into the rifling.
If the bullet is seated to touch the rifling, you will see a higher level of pressure needed to start the bullet moving down the bore, but still it is far less then 20,000 lbs of pressure with most bullets.
In the rounds I deal with the most, large capacity, small to medium small bore wildcats and commerical cartridges, I would say at least 50% of the powder charge passes through the throat in its unburt state.
Why do I think this, Take a round like my 257 Allen Mag with say a 100 gr Ballistic Tip. Load her up to full tilt in a 30" pipe and you will rip that little pill out at near 4100 fps.
If you put a white tarp down and shoot over it, you will find a significant amount of unburnt powder that exits the muzzle when a round is fired. How can this be. Well for starters, the 257 AM is not designed for 100 gr bullets and as such is extremely inefficent at burning 100 gr of powder even in a 30" tube.
Change that bullet to a 156 gr ULD RBBT and the amount of unburnt powder will be much less but some will still be present after each shot.
It is not my opinion that alot of the powder passes through the throat, it is a simple fact that this occurs. It is also a fact that this powder burning in the throat section of the barrel and the first few inches of the beginning of the bore causes alot of heat cracking in the barrel. This is not the case with the chamber area because the chamber steel is protected by the case itself but heat cracking can clearly be seen with a bore scope on any area that is not protected by the case.
As far as flame cutting, yes this can be an issue but generally only in factory rifles. Anyone that has designed a reamer for a high intensity round designed for high performance will design the reamer to cut a throat diameter that is no more then 0.0005" larger then bullet diameter. Thats only 1/4 of a thousandths of an inch on all sides of the bullet, that ain't much room for gas to get by, certainly some does but not enough to cause a flame cutting situation.
Also, may of the loads used by hunters and shooters on this board have the bullets seating to lightly touch or drive slightly into the rifling when a the round is seated, depending on the Ogive of the bullet and the throat angle, seating a bullet in this manor will basically seal off the rifled portion of the bore to the hot gases right from the start.
Yes a minute amount will get by still but again, not enough to do any damage from flame cutting.
Even small capacity rounds such as say a 22-250 will have unburnt powder for several inches down the bore. Again, not my opinion, simple facts.
Heres a simple test you can do. Take two powders, one a ball powder, one a stick powder. Weigh out 100 gr of each.
Put a powder funnel over the stick powder canister and pour all the powder in at one time. See how long it takes for the stick powder to funnel into the canister. More then likely it will bind up and stop its flow unless you tap on the funnel to keep the powder moving.
Now do the same with the 100 gr of ball powder with the funnel over its canister. You will see that every time, the ball powder passes freely through the funnel with no stopages at all, ever. WHy, because the frictional coefficent between each granual of powder is dramatically less with ball powder then it is with stick powders.
If you have ever witnessed a powder bridging situation you know what I am taking about. When this happens, the stick powders acutally do get locked inside the case and pressures spike to extremely dangerous levels. That is what happens with all of the powder burns inside the case itself. Not something we want in out high intensity rifles.
Even extremely small case capacity rounds will start the bullet moving long before the powder is burnt. Case in point is the 22 Hornet. If you use to hot of a primer in a Hornet with light bullets, in a large percentage of the time, accuracy will be sub par. This is because the powder begins the bullet moving before the powder fully ignites.
In most cases, if you use the mildest primer you can get in this round, accuracy will on average improve dramatically because it allow the powder to ignite before starting the bullet down the bore, but still, the bullet is well into the rifling before the powder is fully burnt.
One other test you can do at home if you have ever pulled bullets from test loads. If we needed 50-65,000 psi to move a bullet, we would never be able to pull a bullet from a case with the pullers we use today. Again, a bullet will move with only a few thousand lbs of pressure and by the time the chamber pressure has peaked, the bullet is well down the barrel just as SS7mm has accurately stated.
I would be interested to see the SS7mm do that same chart for his 7mm AM with the 200 gr ULD RBBT and see how many inches of barrel it takes to burn that 105 gr powder charge?
No flaming intended here at all, just want to say that what I said was not opinion, it is proven fact.
Allen Precision Shooting
Home of the Allen Magnum, Allen Xpress and Allen Tactical Wildcats and the Painkiller Muzzle brakes.
50,sorry it took so long to reply i've been away for a while.the main point here is throat erosion comparing stick to ball powders.i don't agree with some of your theories at all and i know some of them are not "facts" as you call them.
a bullets ogive touching the rifling doesn't hardly seal off anything. what is the percentage of bearing surface of the top of the lands compared to the bottom of the grooves in one of those 3 groove barrels?that's the only place where any gases are sealed off.of course the farther the bullet is pushed into the lands,the more it will be sealed off. also a 1/4 thousands is plenty of room for hot gases to escape by creating a cutting torch effect.i agree that the smaller this number, the smaller the amount of erosion.if hot gases escaping around the bullet while it is getting started into the lands aren't much of a reason for throat erosion,why do banded style of bullets get at least twice the throat life?
the point where you guys are wrong is this peak chamber pressure thing.peak pressure occurs getting the bullet started. by this i mean getting the bullet fully engraved into the rifling.i don't know what it takes to start the bullet moving and i agree it's different for bullets just touching, jammed into the lands, or way off the lands. but the peak pressure occurs getting the full length of the bearing surface engraved into the rifling.once it is completely engraved in the rifling it starts accellerating very fast and chamber pressure goes down.peak pressure is not 3 or 4 inches down the barrel.peak pressure at 3 or 4" will happen if you use a banded style of bullet but i assume we are talking about conventional jacketed lead bullets.might i suggest you start looking at actual pressure curves developed when testing bullets instead of quick load which is simply crunching a bunch of theoretical numbers.
i agree that chunks of powder do fly into the throat causing erosion. i just don't think it's anywhere near as much as stated earlier and i don't remember seeing anything proving this.my understanding is ball powders burn cooler and this is the main reason for longer throat life.
I find this very interesting as I really enjoy internal and external ballistics, although I donít have any exotic testing equipment and just some varied software.
In reference to your statement:
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the point where you guys are wrong is this peak chamber pressure thing.peak pressure occurs getting the bullet started. by this i mean getting the bullet fully engraved into the rifling.i don't know what it takes to start the bullet moving and i agree it's different for bullets just touching, jammed into the lands, or way off the lands. but the peak pressure occurs getting the full length of the bearing surface engraved into the rifling.once it is completely engraved in the rifling it starts accellerating very fast and chamber pressure goes down.peak pressure is not 3 or 4 inches down the barrel
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In playing with my QuickLoad software and also in looking at the pressure trace equipment on the RSI site, and a few other spots, can you please refer me to the data, or sources, where you have found the above to be the case. I look and look but it seems to me that the traces done with the RSI strain gage and related equipment and software seems to agree with what my QuickLoad software tells me and that is in reference to the distance down the barrel the bullet has traveled when peak chamber pressure occurs.
I would like to add your documented data to my data base but I canít find where you have obtained your data or who has done the testing. Can you list it so I can read it, as I would find this very interesting.
Everything I find says that it takes far less than your stated pressure to initiate bullet travel and get the bullet fully into the bore, so if you could, please list where your pressure figures are documented and or proven so I can read about it.
Also one other question. You stated that
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the pressure builds up to 50 or 65k to get the bullet started
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According to your data and tests what would happen if the maximum pressure was only, say, 35,000 psi ?? Just curious since your data seems to document that it takes 50,000-65,000 psi to get the bullet started.
Dick, i was reading a magazine article quite a few years ago and they were talking about this very issue.graphs were shown just like the one you posted with several different bullet/cartridge combinations.these were actual pressure readings from the testing done at hornady.in each case the peak pressure was at the moment of complete engraving of the lands into the bullet.i found this interesting so i called the sierra hotline, which was fairly new, and they concurred.i can't give you the magazine name or even the year, but it's been a while so last week,while i was on vacation,i called the sierra hotline to see if i remembered this correctly.they seem to agree with me.my third source would be Gerard Shultz of GS custom bullets.he told me this was another major difference between conventional jacketed lead bullets and his banded style.that's why his bullets will get at least twice the throat life.the primer will engage the bullet in the rifling sealing it off and not getting the flame cutting of the throat area that happens getting a jacketed bullet started into the lands.
as far as the 35 or 50 or 65k pressure thing.i was just using 50-65k as a general number being that's what most cartridges peak at.if a cartridge peaks at 35k so be it.my point is that the peak pressure,whatever it is,happens getting the bullet completely engraved in the rifling.