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.