How Does Barrel Length Affect Velocity?

Lately I’ve been curious about the performance of my 10.5” AR pistol barrel chambered in 5.56.  Specifically, I wanted to know how far the round could travel while still being considered ballistically-acceptable for personal defense.  Additionally, I wanted to know how much of a difference there was in terms of drop-off at a given distance as compared to a standard 16” barrel.  In this article, I will focus on the velocity difference by barrel length.  I will look at ballistic performance in a separate write-up.

 

I’m sure you’ve heard, as I have, that for every inch of barrel length you lose (or gain) it equates to a difference in muzzle velocity of about 50 feet-per-second, but how accurate is that figure? 

Is that simply the equivalent of an old wives’ tale?  Is that only for a specific caliber, maybe a hunting one like the tried-and-true .308 Winchester?  Also, what does a 50 fps drop in muzzle velocity equate to in terms of drop-off at a given distance (let’s use 500 yards)?  What about 100 fps?  200?

 

In an effort to educate myself with real-world, first-hand knowledge, I borrowed a chronograph and took to shooting anything I had chambered in .22 LR, 9mm, and .223 Remington.  I’ve compiled the data by caliber to make it easier to compare apples to apples.

 

Let’s start with the .22 LR.  I was using a few different ammunition types out of a couple different guns.  The first was a 16”-barreled Smith and Wesson M&P 15-22 rifle.  The other was a 4”-barreled Smith and Wesson M&P 22 Pistol.  All testing was done using a Griffin Optimus suppressor, though I don’t believe that figured into the velocities recorded.  Even if it did, the suppressor was configured the same for both the rifle and pistol testing meaning any influence was the same for both guns.  The data I collected is in the chart below for reference.

 

Gun S&W 15-22 Rifle S&W 15-22 Rifle S&W 15-22 Rifle S&W 15-22 Rifle S&W 15-22 Rifle
Caliber .22 Long Rifle .22 Long Rifle .22 Long Rifle .22 Long Rifle .22 Long Rifle
Ammo 40 Grain PHP
Winchester Bulk Pack
36 Grain JHP
CCI Mini Mag
32 Grain JHP
CCI Stinger
40 Grain LRN
CCI Standard Velocity
40 Grain LRN
CCI Quiet
Shot 1 1220 1241 1403 1044 716
Shot 2 1231 1261 1466 1035 644
Shot 3 1237 1257 1496 1064 667
Shot 4 1220 1253 1479 1037 630
Shot 5 1262 1198 1423 1033 700
Max 1262 1261 1496 1064 716
Min 1220 1198 1403 1033 630
Average 1234 1242 1453 1043 671
Std Dev 17 26 39 13 36

 

Gun S&W 22 Pistol S&W 22 Pistol S&W 22 Pistol S&W 22 Pistol S&W 22 Pistol
Caliber .22 Long Rifle .22 Long Rifle .22 Long Rifle .22 Long Rifle .22 Long Rifle
Ammo 40 Grain PHP
Winchester Bulk Pack
36 Grain JHP
CCI Mini Mag
32 Grain JHP
CCI Stinger
40 Grain LRN
CCI Standard Velocity
40 Grain LRN
CCI Quiet
Shot 1 997 1052 1153 853 599
Shot 2 997 1057 1123 897 589
Shot 3 1037 1088 1176 889 602
Shot 4 1064 1151 1180 919 582
Shot 5 1034 1077 1174 883 594
Max 1064 1151 1180 919 602
Min 997 1052 1123 853 582
Average 1026 1085 1161 888 593
Std Dev 29 40 24 24 8

 

The first thing I noticed was the advertised velocities on the ammo boxes were not always achieved using common barrel lengths.  For instance, the “Stinger” ammo stated it should be around 1,640 fps, but I only achieved an average of 1,453 fps.  That’s nearly a 200 fps difference!  The rest seemed to be within 50 fps of their advertised velocities out of the 16” barrel.  Moving to the 4” barrel, the average velocity decrease I recorded was 178 fps.  Now, if the “50 fps-per-inch” were true, I should have seen an average decrease of approximately 600 fps (12” x 50 fps).  I think the reason for the lower number was because the powder burn rate is faster for .22 LR than for typical rifle powders.  Also, if you’re looking for an ammo that you can use through a suppressor out of a pistol that is subsonic, you might be able to get away with a bulk pack.

 

Now, moving on to the 9mm.  The suppressor was used on the Smith and Wesson M&P9 and the CZ Scorpion.  The Glock 43 and Sig P320 were shot with factory, non-threaded barrels.  The ammunition I was using for this test was a “home brew”.  I reload my own using the information detailed in the “Ammo” row, with a COAL of 1.355”.

 

Gun Glock 43 Sig P320 M&P 9 CZ Scorpion
Caliber 9mm Luger 9mm Luger 9mm Luger 9mm Luger
Ammo 115 Grain FMJ
4.8 Grains CFE Pistol
115 Grain FMJ
4.8 Grains CFE Pistol
115 Grain FMJ
4.8 Grains CFE Pistol
115 Grain FMJ
4.8 Grains CFE Pistol
Shot 1 944 949 1048 1139
Shot 2 904 961 1039 1134
Shot 3 963 981 1079 1134
Shot 4 961 982 1076 1178
Shot 5 983 988 994 1158
Max 983 988 1079 1178
Min 904 949 994 1134
Average 951 972 1047 1149
Std Dev 30 16 34 19

 

The longest barrel I used was on the CZ Scorpion at 7.75” in length.  This was the only gun that could develop enough pressure to cause the 115 grain projectiles to become supersonic.  The velocity decrease when moving to the 5” M&P9 barrel was only 102 fps slower.  This is right on point with the saying (2” x 50 fps), so maybe there is something to this.  The G43 and P320 have a 3.4” and 4” barrel, respectively, and performed as estimated using the old tale.

 

Let’s look at the .223 Remington data I collected.  Note that all of the .223 rounds shot through the chronograph were the same across all of the guns.  These are, again, reloads.  My recipe is for 27 grains (+/- .1 gr) of CFE 223 powder with CCI Small Rifle Magnum primers using Hornady 55 grain FMJBT bullets seated to a COAL of 2.222”.  The SDMR and AR Pistol guns were shot using the suppressor in rifle configuration, whereas the Class AR was shot using the Effin-A tunable muzzle device.  Barrel lengths are: SDMR = 18”, Class AR = 16”, AR Pistol = 10.5”.

 

Gun SDMR AR-15 Class AR-15 AR-15 Pistol
Caliber .223 Remington .223 Remington .223 Remington
Ammo 55 Grain FMJBT
27 Grains
CFE 223
Magnum Primer
55 Grain FMJBT
27 Grains
CFE 223
Magnum Primer
55 Grain FMJBT
27 Grains
CFE 223
Magnum Primer
Shot 1 3018 2913 2510
Shot 2 3023 2982 2517
Shot 3 3028 2941 2496
Shot 4 2954 2907 2526
Shot 5 3011 2926 2482
Max 3028 2982 2526
Min 2954 2907 2482
Average 3007 2934 2506
Std Dev 30 30 17

 

The highest average velocity recorded was with the longest barrel and the slowest was with the shortest barrel.  It’s also worth noting that I should have seen 100 fps drop from the SDMR to the Class AR, but I only saw an average of 73 fps.  Additionally, I should have seen a drop of 400 fps from the SDMR to the AR Pistol (8” x 50 fps), but I averaged 501 fps.  Also, from the Class AR to the AR Pistol I should have seen an average velocity decrease of 300 fps (6” x 50 fps), but I experienced 428 fps.

 

Interestingly, it seems as though the pistol powders followed the “50 fps per inch of barrel” estimate in a mostly linear fashion.  The rifle powders, however, do not seem to follow this estimate.  They especially don’t follow it as you get below the 16” barrel length.  If I can get access to some, I would love to run the same test on 14.5”, 12”, and 7” barrels.

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