BS Busters Number 87: The Myth of the 18 Inch Barrel Velocity
BS Busters Number 87: The Myth of the 18 Inch Barrel Velocity
The BS: “20 inch barrels for the AR-15 are obsolete because 18 inch barrels produce muzzle velocities that are only a few feet per second less than the muzzle velocities of 20 inch barrels.”
The Facts: If you’re going to make a valid comparison between the muzzle velocity of an 18 inch barrel and a 20 inch barrel you need to use the same type of barrels, from the same manufacturer, made from the same steel, with the same type of rifling and twist-rate and the same chamber. Comparing the muzzle velocity of a stainless-steel 18 inch barrel that has a 223 Wylde chamber, polygonal rifling and 1:8” twist, to that of a 20 inch chrome-lined CMV barrel that has a 5.56mm NATO chamber, standard rifling and a 1:7” twist is asinine; yet that’s just what the Internet Commandos and other self-anointed SMEs do in order to make their ridiculous claim.
Secondly, in order to make a valid comparison, you need to chronograph a large enough number of shots from each barrel to remove the variable of the velocity deviations of the ammunition itself from the equation; piss-ant five-shot strings don’t cut it. Using quality ammunition with a low standard deviation is also a plus.
Thirdly, in order to make a valid comparison, you need to use a chronograph that has verifiable readings. Most of the non-industrial chronographs currently available simply aren’t up to the task; with one notable exception, that exception being the Oehler 35P chronograph with proof-screen technology.
The Oehler 35P chronograph is actually two chronographs in one package that takes two separate chronograph readings for each shot and then has its onboard computer analyze the data to determine if there is any statistically significant difference between the two readings. If there is, the chronograph flags the shot to let you know that the data is invalid. Obviously, testing should also be conducted under the same atmospheric conditions.
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The Data. On the same day, following the methodology described above, I conducted a muzzle velocity comparison between a stainless-steel Noveske 20 inch DCM barrel that has polygonal rifling, a 1:7” twist and a Noveske Match Mod 0 chamber, to that of a stainless-steel Noveske 18 inch SPR barrel that also has polygonal rifling, a 1:7” twist and a Novekse Match Mod 0 chamber. An Oehler 35P chronograph with proof screen technology was used for obtaining and recording the velocity data. (Velocities listed below are muzzle velocities, as calculated from the instrumental velocities using Oehler’s Ballistic Explorer software.
20 inch Noveske barrel
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18 inch Noveske barrel
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Thirty shots of Hornady’s 5.56mm 75 grain TAP T2 were fired over the chronograph from each barrel described above. (Thirty occurrences of the subject being studied are what the statisticians like to refer to as a “large sample.”) The shots were fired in a single-loading manner from a Bob-sled to allow the bolt to lock-back and the chamber to cool between each shot. A delay of 30 seconds between the loading of each round as well as the round being fired within three seconds of being loaded was performed in order to mitigate the effects of “chamber soaking” on the velocity readings.
The average muzzle velocity of the 30 shots of TAP T2 fired from the 20 inch Noveske barrel was 2885 FPS. The average muzzle velocity of the 30 shots fired from the 18 inch Noveske barrel was 2779 FPS; for a difference of 106 FPS.
I also conducted the same test in the same manner as above using the Black Hills 5.56mm 50 grain TSX ammunition. Thirty shots of the Black Hills load fired from the 20 inch barrel produced a muzzle velocity of 3414 FPS. The thirty shots fired from the 18 inch barrel had a muzzle velocity of 3305 FPS; for a difference of 109 FPS.
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Another valid method of comparing the muzzle velocities of different barrel lengths is to use the same single barrel for testing; that is, start the chronographing with the barrel at a length of 20 inches, then remove two inches from the muzzle of the barrel for a length of 18 inches, recrown the barrel and then chronograph at that length. I’m not about to lop off pieces from the muzzle of my Noveske barrel, but the good folks at Accuratereloading.com have conducted just such a test.
Using the barrel of a Sako bolt action rifle, they chronographed 70 shots of assorted 223 Remington 52 grain hand-loads from each length of the sectioned barrel. The difference in the average muzzle velocities of the 20 inch section of barrel and the 18 inch section of the barrel was 99 FPS.
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