New Build Recommendations?

[jimmykan]

Is there a ballistic performance difference between 18" and 20" barrels? Yes.

Does it make a practical difference? Let's see...

Assumptions: Shooting 5.56 NATO 77 grain ammunition (e.g. the Black Hills 77 grain Sierra Matchking load)

Muzzle Velocities (observed from my 18" and 20" barreled AR-15s and supported by QuickLOAD simulations):

18 inch barrel: 2670 fps
20 inch barrel: 2750 fps


So a difference of about 80 fps. Does this matter down range?

Plugging these scenarios into JBM Ballistics Calculator:

Sierra, .224 cal, 77 gr, Matchking (Litz), 0.190 G7 BC
2670 fps, 100 yard ZERO, 1500 ft ASL, STD ATM, 10 mph WIND
Elevation Drop at 600 yards: 5.0 mil (50 clicks)
Elevation Drop at 700 yards: 6.7 mil (67 clicks)
Elevation Drop at 800 yards: 8.6 mil (86 clicks)
Windage Correction at 600 yards: 1.9 mil (19 clicks)
Windage Correction at 800 yards: 2.9 mil (29 clicks)
Sound Barrier Crossing: 850 yards

Sierra, .224 cal, 77 gr, Matchking (Litz), 0.190 G7 BC
2750 fps, 100 yard ZERO, 1500 ft ASL, STD ATM, 10 mph WIND
Elevation Drop at 600 yards: 4.7 mil (47 clicks)
Elevation Drop at 700 yards: 6.2 mil (62 clicks)
Elevation Drop at 800 yards: 8.0 mil (80 clicks)
Windage Correction at 600 yards: 1.8 mil (18 clicks)
Windage Correction at 800 yards: 2.7 mil (27 clicks)
Sound Barrier Crossing: 875 yards

The faster trajectory will be a LITTLE bit more forgiving when dialing in elevation drop, in case you are off on your range estimation.

Difference in windage correction is negligible. I can't read wind within +/- 5 mph, maybe not even +/- 10 mph. I just know it's blowing when my chronograph topples over.