FPS gain per inch
- Thread starter dragman
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Keith Glasscock
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I was recently directed to an article in the Spring 2006 issue of the journal of the NRA (UK). There is an article in there about 155 and 200 grain sierra bullets as a barrel was shortened from 35-23"
While the methodology wasn't perfect, it was darn good. The same load was fired through the barrel as the length was cut back in 2" increments.
The results showed that a 155 grain bullet powered with N140 would lose 80 fps when cutting back from 30 to 24"
The 200 grain would lose 50-60 fps depending on powder (N550 lost more than N150)
The limitations to this data are significant. During testing, the temperature on the range rose 5C (9F). As such, the shorter barrel lengths might have been producing a higher MV. Even the most sensitive of "reasonable" powders only makes 1.25 fps or so per degree F, So I'd have to guess that the data is only skewed no more than 10 fps. The big question is how the chamber pressure changed during testing. I can imagine that the ammo did not produce the same peak pressures at every length. The magnitude of that potential error (if there is one at all) is questionable.
The results are not entirely inconsistent with what I've seen elsewhere (look at the bottom of the 308 page here for QL estimates).
250 fps is unquestionably too big for similar pressures. The 20 fps per inch is a bit generous past 26"
On the other hand, barrels are individuals. Two identical barrels can give differing MV's by 100 fps or more.
While the methodology wasn't perfect, it was darn good. The same load was fired through the barrel as the length was cut back in 2" increments.
The results showed that a 155 grain bullet powered with N140 would lose 80 fps when cutting back from 30 to 24"
The 200 grain would lose 50-60 fps depending on powder (N550 lost more than N150)
The limitations to this data are significant. During testing, the temperature on the range rose 5C (9F). As such, the shorter barrel lengths might have been producing a higher MV. Even the most sensitive of "reasonable" powders only makes 1.25 fps or so per degree F, So I'd have to guess that the data is only skewed no more than 10 fps. The big question is how the chamber pressure changed during testing. I can imagine that the ammo did not produce the same peak pressures at every length. The magnitude of that potential error (if there is one at all) is questionable.
The results are not entirely inconsistent with what I've seen elsewhere (look at the bottom of the 308 page here for QL estimates).
250 fps is unquestionably too big for similar pressures. The 20 fps per inch is a bit generous past 26"
On the other hand, barrels are individuals. Two identical barrels can give differing MV's by 100 fps or more.
That's a really good question. I have done some searching, and the answers are all over the map. I just found this on The High Road:
Just found this information from the following web site: http://www.chuckhawks.com/rifle_barrel.htm
The 2001 Edition of the Shooter's Bible states, in the introduction to the Centerfire Rifle Ballistics section, "Barrel length affects velocity, and at various rates depending on the load. As a rule, figure 50 fps per inch of barrel, plus or minus, if your barrel is longer or shorter than 22 inches." But they do not say what category of load to which this 50 fps average pertains.
Jack O'Connor wrote in The Rifle Book that, "The barrel shorter than standard has a velocity loss which averages about 25 foot-seconds for every inch cut off the barrel. Likewise, there is a velocity gain with a longer barrel." He went on to illustrate this using a .30-06 rifle shooting 180 grain bullets as an example, so his estimate was obviously for rifles in that general performance class.
Other authorities have tried to take into account the different velocity ranges within which modern cartridges operate. The Remington Catalog 2003 includes a "Centerfire Rifle Velocity Vs. Barrel Length" table that shows the following velocity changes for barrels shorter or longer than the test barrel length:
MV 2000-2500 fps, the approximate change in MV per 1" change in barrel length is 10 fps.
MV 2500-3000 fps, the approximate change in MV per 1" change in barrel length is 20 fps.
MV 3000-3500 fps, the approximate change in MV per 1" change in barrel length is 30 fps.
MV 3500-4000 fps, the approximate change in MV per 1" change in barrel length is 40 fps.
The 45th Edition of the Lyman Reloading Handbook also has a table showing Center Fire Rifle Velocity Vs. Barrel Length. Their figures apply to barrels between 20 and 26 inches in length and agree with the Remington figures. The Lyman table shows the following approximate velocity changes:
For rifles with muzzle velocities in the 1000-2000 fps range, the change in velocity for each 1" change in barrel length is 5 fps.
For rifles with muzzle velocities in the 2001-2500 fps range, the change in velocity for each 1" change in barrel length is 10 fps.
For rifles with muzzle velocities in the 2501-3000 fps range, the change in velocity for each 1" change in barrel length is 20 fps.
For rifles with muzzle velocities in the 3001-3500 fps range, the change in velocity for each 1" change in barrel length is 30 fps.
For rifles with muzzle velocities in the 3501-4000 fps range, the change in velocity for each 1" change in barrel length is 40 fps.
The 43rd edition of the Lyman reloading Handbook gave some concrete examples of velocity loss for specific calibers and loads. The Lyman technicians chronographed some high velocity cartridges in rifles with barrels ranging in length from 26" down to 22" with the following results:
The average loss for the .243 Win./100 grain bullet was 29 fps per inch.
The average loss for the .264 Win. Mag./140 grain bullet was 32 fps per inch.
The average loss for the .300 H&H Mag./220 grain bullet was 25 fps per inch.
For standard high intensity cartridges in the same test, the Lyman technicians chronographed the cartridges in barrel lengths ranging in length from 24" down to 20" with the following results:
The average loss for the .270 Win./130 grain bullet was 37 fps per inch.
The average loss for the .270 Win./150 grain bullet was 32 fps per inch.
The average loss for the .300 Sav./180 grain bullet was 17 fps per inch.
The average loss for the .30-06/180 grain bullet was 15 fps per inch.
The average loss for the .35 Rem./200 grain bullet was 11 fps per inch.
After a bunch of disclaimers, the Lyman people concluded, "The rule of thumb is that high speed, high pressure cartridges shed more speed in short barrels than do the low speed, large bore types." It's funny, but that is what I had suspected all along!
Just found this information from the following web site: http://www.chuckhawks.com/rifle_barrel.htm
The 2001 Edition of the Shooter's Bible states, in the introduction to the Centerfire Rifle Ballistics section, "Barrel length affects velocity, and at various rates depending on the load. As a rule, figure 50 fps per inch of barrel, plus or minus, if your barrel is longer or shorter than 22 inches." But they do not say what category of load to which this 50 fps average pertains.
Jack O'Connor wrote in The Rifle Book that, "The barrel shorter than standard has a velocity loss which averages about 25 foot-seconds for every inch cut off the barrel. Likewise, there is a velocity gain with a longer barrel." He went on to illustrate this using a .30-06 rifle shooting 180 grain bullets as an example, so his estimate was obviously for rifles in that general performance class.
Other authorities have tried to take into account the different velocity ranges within which modern cartridges operate. The Remington Catalog 2003 includes a "Centerfire Rifle Velocity Vs. Barrel Length" table that shows the following velocity changes for barrels shorter or longer than the test barrel length:
MV 2000-2500 fps, the approximate change in MV per 1" change in barrel length is 10 fps.
MV 2500-3000 fps, the approximate change in MV per 1" change in barrel length is 20 fps.
MV 3000-3500 fps, the approximate change in MV per 1" change in barrel length is 30 fps.
MV 3500-4000 fps, the approximate change in MV per 1" change in barrel length is 40 fps.
The 45th Edition of the Lyman Reloading Handbook also has a table showing Center Fire Rifle Velocity Vs. Barrel Length. Their figures apply to barrels between 20 and 26 inches in length and agree with the Remington figures. The Lyman table shows the following approximate velocity changes:
For rifles with muzzle velocities in the 1000-2000 fps range, the change in velocity for each 1" change in barrel length is 5 fps.
For rifles with muzzle velocities in the 2001-2500 fps range, the change in velocity for each 1" change in barrel length is 10 fps.
For rifles with muzzle velocities in the 2501-3000 fps range, the change in velocity for each 1" change in barrel length is 20 fps.
For rifles with muzzle velocities in the 3001-3500 fps range, the change in velocity for each 1" change in barrel length is 30 fps.
For rifles with muzzle velocities in the 3501-4000 fps range, the change in velocity for each 1" change in barrel length is 40 fps.
The 43rd edition of the Lyman reloading Handbook gave some concrete examples of velocity loss for specific calibers and loads. The Lyman technicians chronographed some high velocity cartridges in rifles with barrels ranging in length from 26" down to 22" with the following results:
The average loss for the .243 Win./100 grain bullet was 29 fps per inch.
The average loss for the .264 Win. Mag./140 grain bullet was 32 fps per inch.
The average loss for the .300 H&H Mag./220 grain bullet was 25 fps per inch.
For standard high intensity cartridges in the same test, the Lyman technicians chronographed the cartridges in barrel lengths ranging in length from 24" down to 20" with the following results:
The average loss for the .270 Win./130 grain bullet was 37 fps per inch.
The average loss for the .270 Win./150 grain bullet was 32 fps per inch.
The average loss for the .300 Sav./180 grain bullet was 17 fps per inch.
The average loss for the .30-06/180 grain bullet was 15 fps per inch.
The average loss for the .35 Rem./200 grain bullet was 11 fps per inch.
After a bunch of disclaimers, the Lyman people concluded, "The rule of thumb is that high speed, high pressure cartridges shed more speed in short barrels than do the low speed, large bore types." It's funny, but that is what I had suspected all along!
Here is a link to a white paper documenting a test procedure similar to the one Busdriver described. It uses a 7.62 X 54R in a Moisin Nagant. So it should be fairly similar to what you might experience with a .308 Win. Bottom line: the author removed 12 inches of barrel and found the differences in the average MV to be 306 fps, 25.5 fps per inch.
http://honors.usf.edu/documents/Thesis/U82488180.pdf
http://honors.usf.edu/documents/Thesis/U82488180.pdf
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