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.
