Fiftydriver
Official LRH Sponsor
Well, I have been wanting to do this experiment for a long time. Over the years, I have seen to many short throated rifles put up velocity numbers that would just make many out there question the results. So, I have always wanted to do a head to head comparision to see what throat length really did.
Now, probably the very best way to do this would be to build a rifle chambered with a short throat, do load development with a pressure guage to get pressure data and then record velocity and then cut the throat longer and repeat and see what the results were when the longer throated chamber was loaded to the same exact pressure.
Well, I simply do not have time to do that right now but I had a unique opportunity fall into my hand the past couple weeks. Last week I finished up one of my Raptor LRSS rifles in 338 Allen Xpress for a customer. 30" Lilja, 1-10 twist. Breech bore diameter was 0.3304", muzzle diameter 0.3302".
This rifle was a single shot and the throat length was such that a 300 gr berger OTM was seated to 3.950" to touch the lands. I used RL33 for this test and started at 100.0 gr and worked up 1/2 grain at a time until the case heads went from absolutely no ejector ring of any kind to just the SLIGHTEST hint of a ring with a 1/2 grain increase. To confirm this, once I got to this point, to confirm I loaded up three rounds at that load and the one load at less by 1/2 grain to make sure that this was really the point where the ejector ring just started to show and not just a single case that was a bit softer then the rest of the cases.
With this rifle, that point was going from 109.5 grains to 110.0 grains which produced an average velocity of 3041 fps.
Today I was able to do load development on another Raptor LRSS with Lilja 1-10 twist, 30" barrel. Breech diameter was the exact same 0.3304" and muzzle diameter was also the exact same 0.3302".
The only difference was that that this rifle was a repeater set up to use a Badger Ord detachable magazine system with the 300 gr berger OTM seated to 3.740" OAL.
In both tests, the same lot of powder was used. Same lot of brass, same lot of bullets.
Now, I was expecting that RL33 may be to slow burning for this short OAL with the big 300 gr OTM taking up to much case volume to reach top working pressures. Still for the sake of getting the data, I wanted to give it a try even though I thought that Retumbo would be a better match for this rifle.
Because of the reduced load capacity, I started at 95.0 grains of RL33 which proved to be VERY mild. Again, the test was to increase load 1/2 grains at a time until the faintest ejector ring just became present, same as the first riflle. To be sure not to influence the results while I was performing the test, I did not shoot the rounds over the chronograph. Instead I simply shot the rounds until I got to the point where the ejector ring JUST started to appear.
The first test was done at 70 degree temp and today was within a couple degrees of that test temperature.
This short throated rifle topped out at 107.0 grains of RL33. That was not a surprise, the shorter OAL has to reduce case capacity. The suprising thing came when I chronographed that 107.0 gr load. It average velocity was 3037 fps. 4 fps less then the long throated barrel with what appeared to be nearly identical pressures.
So..............
We have always been told that you could take a round and by increasing the length of the throat, you could increase velocity and while this may be true to some degree but in this case, it clearly shows that throat length is not all its cracked up to be. Just because you add 0.200" of length to your OAL in ammo, do not expect that you will automatically increase in velocity.
In this test, when both 3.740" and 3.950" ammo were loaded to apparently the same chamber pressure, velocity was nearly identical.
One interesting note however, the 3.740" length had around a 104% load density. The 3.950" length was around 98% load density. Yet, chamber pressure was as close as I could get it with the means I was testing with.
Now, I am sure some of you will simply dismiss this by saying that obviously the shorter round was simply loaded to higher pressures...... All I can say that is Each of these tests produced the exact same FAINT shadow ejector ring on the case head with only a 1/2 increase in charge weight. The load before with each load, there was NO shadow ring of any kind.
I have done similar tests with my 7mm Allen Magnum when I was doing the very first load development for that wildcat and using the old Wildcat Bullet 200 gr ULD RBBT seated at 3.600" and at 3.900" with very similar results.
I have just seen to many short throated rifles come very close to matching long throated barrels to not think that throat length has less effect on velocity then we may think. My belief is that as long as a given case has the appropriate case capacity to use a specific burn rate of powder to allow 95 to 105% load density with that powder, it will produce very similar top end velocities no matter what the throat length that is used.....
Now, with all this said, I have seen several instances where seating a bullet dramatically longer resulted in a case capacity that allows a round to step to a different burn rate class of powder. In this case, yes, velocity output can be quite different. But using the same powder with same bullets in same length barrels and loaded to same chamber pressures, the difference may not be as large and many of us have been lead to believe.
One note to mention, the throat design in both rifles was identical in that both were 0.3385" diameter and both have a 1-30-00 lead angle.
Take if for what its worth but in this test, throat length did not appear to offer much of anything as far as velocity output potential.
Now, probably the very best way to do this would be to build a rifle chambered with a short throat, do load development with a pressure guage to get pressure data and then record velocity and then cut the throat longer and repeat and see what the results were when the longer throated chamber was loaded to the same exact pressure.
Well, I simply do not have time to do that right now but I had a unique opportunity fall into my hand the past couple weeks. Last week I finished up one of my Raptor LRSS rifles in 338 Allen Xpress for a customer. 30" Lilja, 1-10 twist. Breech bore diameter was 0.3304", muzzle diameter 0.3302".
This rifle was a single shot and the throat length was such that a 300 gr berger OTM was seated to 3.950" to touch the lands. I used RL33 for this test and started at 100.0 gr and worked up 1/2 grain at a time until the case heads went from absolutely no ejector ring of any kind to just the SLIGHTEST hint of a ring with a 1/2 grain increase. To confirm this, once I got to this point, to confirm I loaded up three rounds at that load and the one load at less by 1/2 grain to make sure that this was really the point where the ejector ring just started to show and not just a single case that was a bit softer then the rest of the cases.
With this rifle, that point was going from 109.5 grains to 110.0 grains which produced an average velocity of 3041 fps.
Today I was able to do load development on another Raptor LRSS with Lilja 1-10 twist, 30" barrel. Breech diameter was the exact same 0.3304" and muzzle diameter was also the exact same 0.3302".
The only difference was that that this rifle was a repeater set up to use a Badger Ord detachable magazine system with the 300 gr berger OTM seated to 3.740" OAL.
In both tests, the same lot of powder was used. Same lot of brass, same lot of bullets.
Now, I was expecting that RL33 may be to slow burning for this short OAL with the big 300 gr OTM taking up to much case volume to reach top working pressures. Still for the sake of getting the data, I wanted to give it a try even though I thought that Retumbo would be a better match for this rifle.
Because of the reduced load capacity, I started at 95.0 grains of RL33 which proved to be VERY mild. Again, the test was to increase load 1/2 grains at a time until the faintest ejector ring just became present, same as the first riflle. To be sure not to influence the results while I was performing the test, I did not shoot the rounds over the chronograph. Instead I simply shot the rounds until I got to the point where the ejector ring JUST started to appear.
The first test was done at 70 degree temp and today was within a couple degrees of that test temperature.
This short throated rifle topped out at 107.0 grains of RL33. That was not a surprise, the shorter OAL has to reduce case capacity. The suprising thing came when I chronographed that 107.0 gr load. It average velocity was 3037 fps. 4 fps less then the long throated barrel with what appeared to be nearly identical pressures.
So..............
We have always been told that you could take a round and by increasing the length of the throat, you could increase velocity and while this may be true to some degree but in this case, it clearly shows that throat length is not all its cracked up to be. Just because you add 0.200" of length to your OAL in ammo, do not expect that you will automatically increase in velocity.
In this test, when both 3.740" and 3.950" ammo were loaded to apparently the same chamber pressure, velocity was nearly identical.
One interesting note however, the 3.740" length had around a 104% load density. The 3.950" length was around 98% load density. Yet, chamber pressure was as close as I could get it with the means I was testing with.
Now, I am sure some of you will simply dismiss this by saying that obviously the shorter round was simply loaded to higher pressures...... All I can say that is Each of these tests produced the exact same FAINT shadow ejector ring on the case head with only a 1/2 increase in charge weight. The load before with each load, there was NO shadow ring of any kind.
I have done similar tests with my 7mm Allen Magnum when I was doing the very first load development for that wildcat and using the old Wildcat Bullet 200 gr ULD RBBT seated at 3.600" and at 3.900" with very similar results.
I have just seen to many short throated rifles come very close to matching long throated barrels to not think that throat length has less effect on velocity then we may think. My belief is that as long as a given case has the appropriate case capacity to use a specific burn rate of powder to allow 95 to 105% load density with that powder, it will produce very similar top end velocities no matter what the throat length that is used.....
Now, with all this said, I have seen several instances where seating a bullet dramatically longer resulted in a case capacity that allows a round to step to a different burn rate class of powder. In this case, yes, velocity output can be quite different. But using the same powder with same bullets in same length barrels and loaded to same chamber pressures, the difference may not be as large and many of us have been lead to believe.
One note to mention, the throat design in both rifles was identical in that both were 0.3385" diameter and both have a 1-30-00 lead angle.
Take if for what its worth but in this test, throat length did not appear to offer much of anything as far as velocity output potential.
