Velocity Comparison to barrel Length Testing on AR-15s

[SA Friday]

PLEASE DON'T POST IN THIS THREAD UNTIL I GET EVERYTHING POSTED IN HERE. IT'S GOING TO TAKE A WHILE AND MULTIPLE ENTRIES. I'LL DO A SEPARATE INDIVIDUAL POST SAYING IT'S ALL POSTED WHEN DONE.

Velocity Comparison to Barrel Length on an AR-15 4 Oct 16

The question as to just how much advantage a longer barrel is comes up all the time in a gun store that specializes in building ARs. The question isn’t new. It’s been tested before. Repeating the results from other’s tests isn’t the same as testing the barrels oneself. So, that’s what I did. I went to see what I could learn.

I ran 5 different loads through 7 different ARs of different barrel lengths and recorded the muzzle velocities. Five rounds of each cartridge were shot over a chronograph and the high, low, average, and standard deviation were recorded from each string. Additionally, two of the seven ARs had 16 inch barrels but different twist rates, 1:7 and 1:9. Both were shot to compare velocities against each other. As the equipment was available, I also ran XM193 ammo through suppressors on 5 of the ARs to determine any repeatable change in velocities.

The chronograph used to measure velocity was the Competition Electronics ProChrono Electronic Chronograph on a tripod mount. The chronograph was placed approximately 10 feet in front of the shooting bench and sunscreens were utilized. The weather was sunny all day with no moments of could cover. Shooting was conducted from approximately 1045 to 1430. Slight adjustments to the chronograph were made to ensure no shadows were on the light sensors during testing.

All ARs were function checked and sighted in prior to shooting during testing. No cleaning was conducted before or during testing. Any firearm malfunctions or chronograph malfunctions were resolved on site and testing re-accomplished for the respective string of fire. No accuracy testing was accomplished during testing, but groups were monitored for any apparent inaccuracy as it would possibly point to faulty test results. None were seen during shooting.

The following ammunition was utilized during testing.


Independence

XM193/5.56 NATO

55gr FMJ







Remington

223 Rem

55gr FMJ







Reload

223 Rem

55gr FMJ

Ramshot TAC

Mixed brass



HPR

223 Rem

55gr V-Max







Reload

223 Rem

77gr SMK

Varget

Winchester Brass




All results were transcribed and then transferred into Microsoft Excel spreadsheets. The results were further interpreted into additional spreadsheets to relate relevant conclusions. All tables and trend-lines were produced through Microsoft Excel.

Testing revealed barrels do produce more velocity at the muzzle as they progressively become longer, but at a diminishing return. The additional acceleration as barrels are longer is linearly expressed (See below for linear equation).

[SA Friday]

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[SA Friday]

Standard deviation between strings ranged from 6 to 70 fps. The 70 fps SD was identified as erroneous and one of the five shots in the string was clearly slower than the other four shots. The information was recorded, but corrected to reflect a more accurate representation of the velocity of the string. The anomaly occurred from a 55gr reload out of the 18” Noveske barrel. With the slower round excluded in the average would result in 2976 fps which more accurately reflects the identified trend-line. Regardless, the anomalous round insignificantly impacted the results in this part of the analysis.
Velocities were then subtracted from their immediate shorter barrel and then the velocity gains per inch were calculated.

[SA Friday]

67634

[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.

[SA Friday]

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

[SA Friday]

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]

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[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.

[Great-Kazoo]

Appreciate the write up and time spent testing. IMO for most applications, any 16" bbl should work for the average shooter.

[MarkCO]

This site has a good data set: http://www.ballisticsbytheinch.com/223rifle.html

[Madusa]

Good work! I did something not as intense as yours but it may add to your data. I started out with a 24" barrel (DPMS) with a 1-9 twist. With handloads of SMK 69 g /26g Winchester 748/CCI mag primers. I got average velocity of 3031 fps. Then I cut the barrel down to 18" and ran the same load to get 2932 fps. Also it should be said that these test were not done the same day and external ballistics may be a little different.

[vossman]

This is good shit. Thx for the work.

[SA Friday]

Appreciate the write up and time spent testing. IMO for most applications, any 16" bbl should work for the average shooter.

Agreed. After testing, I'm leaning toward the 14.5" being the optimal barrel length for overall short to mid range use. 16" is just more convenient due to NFA requirements.