Velocity Comparison to barrel Length Testing on AR-15s

[SA Friday]

Attachment 67633

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[SA Friday]

Based on the charting of the results, multiple questionable results can be seen. The velocity results for the 20” barrel was stunted. It’s readily apparent the 20” barrel is slower than all of the other barrels. The 77gr SMK results from the 18” and 20” barrel are slower than expected. There also appears to have been slower than predicted results from the Remington 223 Rem ammo in the 16” barrel string. As it was significant to the linear trend-line, the corrected velocity and subsequent fps per inch was injected into the 16” DD to 18” barrel measurement, denoted in the red font on the above chart. The results reflect just how much variation is involved in ammunition production. Even with the deviation, the result of the analysis of the linear relationship was still clear. As the barrel becomes longer, the acceleration diminishes.

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Adjusted analysis with the 20” barrel results removed. The linear regression is much more apparent.

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X193 out of the 10.3” barrel with a muzzle velocity of 2704 with a 50 yd zero will shoot +/- 2.5” from zero to 170 yds. At 500 yds this combination would have approximate 88 inches of drop and a velocity of 1139 fps. The bullet would go subsonic at 510 yds.

X193 out of the 14.5” barrel with a muzzle velocity of 3033 with a 50 yd zero will shoot +/- 2.5” from zero to 190 yds. At 500 yds this combination would have approximate 63 inches of drop and a velocity of 1353 fps. The bullet would go subsonic at 593 yds.

X193 out of the 18” barrel with a muzzle velocity of 3295 with a 50 yd zero will shoot +/- 2.5” from zero to 215 yds. At 500 yds this combination would have approximately 50 inches of drop and a velocity of 1530 fps. The bullet would go subsonic at 656 yds.

Based on these theoretical ballistics derived from Ballistic AE software, The 3.5 inches of barrel add 8% more muzzle velocity. This relates to an additional maximum point blank range of 25 yds, and a loss of 13 inches of drop at 500 yds. It adds an additional 63 yds to reaching the subsonic barrier.

X193 out of the 24” barrel with a muzzle velocity of 3495 with a 50 yd zero will shoot +/- 2.5” from zero to 235 yds. At 500 yds this combination would have approximately 42 inches of drop at a velocity of 1669 fps. The bullet would go subsonic at 703 yds.

Based on these theoretical ballistics, the 6 inches of barrel from an 18” to a 24” add 5.7% more muzzle velocity. This relates to an additional maximum point blank range of 20 yds, and a loss of 12 inches of drop at 500 yds. It adds an additional 47 yds to reaching the subsonic barrier.

77gr Sierra Match king reloads out of the 14.5” barrel with a muzzle velocity of 2433 with a 50 yd zero will shoot +/- 2.5” from zero to 160 yds. At 500 yds this combination would have approximate 88 inches of drop and a velocity of 1374 fps. The bullet would go subsonic at 696 yds.

77gr Sierra Match king reloads out of the 18” barrel with a muzzle velocity of 2577 with a 50 yd zero will shoot +/- 2.5” from zero to 170 yds. At 500 yds this combination would have approximate 76 inches of drop and a velocity of 1469 fps. The bullet would go subsonic at 752 yds.

77gr Sierra Match king reloads out of the 24” barrel with a muzzle velocity of 2726 with a 50 yd zero will shoot +/- 2.5” from zero to 180 yds. At 500 yds this combination would have approximate 66 inches of drop and a velocity of 1571 fps. The bullet would go subsonic at 808 yds.

[SA Friday]

[SA Friday]

CONCLUSIONS

1. Barrel length in this caliber will continuously accelerate as the barrel length becomes longer.
2. The acceleration gained from longer barrel lengths has a diminishing gain.
3. Out to 300 yds with XM193 ammunition, the difference in ballistics from a 14.5” to 18” barrel is only roughly 3.5 inches. Although not reflected, the same calculations were done with Remington 223 Rem 55gr FMJ ammunition and determined the ballistics from a 14.5” to an 18” barrel is only roughly 3.1 inches. The combined difference between the 223 Rem and XM193 is only roughly 6 inches.
4. Out to 300 yds with XM193, the difference in ballistics from a 10.3” to 18” is roughly 9 inches.
5. It’s possible the 77gr SMK reload round is optimally shot from no longer than an 18” barrel as there was very limited return after 18 inches. This will take more testing and the limited results here could have anomalies in it due to limited rounds fired during velocity testing.
6. My 20” HBAR is slow as fuck.
7. The difference in velocity from a 1:9 vs 1:7 twist barrels was roughly 2% and impacted more by bullet construction than the twist.
8. The velocity difference of XM193 with and without a suppressor was well within the limits of standard deviation for the various ammunition and barrels and essentially zero.

[SA Friday]

OK, that should be it short of a few pics which are really just fluff and not relevant to the info. Post away if you want. I tried to get it on here as best as possible. It's all from a word doc, and this was the easiest way I could figure out to get it posted. Sorry it's so choppy.

[SA Friday]

Before it's asked, "Why didn't you use XXXX ammo." Well, I didn't have enough 69gr loaded and I don't have M855 as I find it shitty at best. So I didn't add those to the testing. After analysis, I'm not sure they would have added much to the results anyway. What the testing would have really benefitted from was more shots per sequence, but this was fairly expensive as it was.