Connecticut Preparedness

[nvisn]

My boss is looking to build a 600 yard AR-15 using commercially available 55-62 grain ammo. What barrel length and twist would be ideal for this type of application. Thanks for any input.

[JohnFH]

1/9 20 inch, pretty much an HBAR, that is basicly what they were designed for.

[hayes1966]

My RRA national match is 1/8 and likes 69gr.

[MadSmith]

http://ammo.ar15.com/ammo/project/perf_twists.html

[nvisn]

Any thoughts on lined/unlined/stainless barrels?

[MadSmith]

Stainless steel barrels are much more resistant to corrosion, but the metal is softer than the equivalent non-stainless barrel.

Chrome lined barrels are also less prone to corrosion, but the lining is known to degrade precision, as the chrome deposition cannot be made even, and they wear unevenly as time goes on.

Hammer forged barrels are easier to make, and with proper stress relieving, provide good cost effective solutions.

Cut barrels are much more expensive and rarer, and more prone to error in manufacturing. It's an older technology. However, if done well, they provide better precision than hammer forged barrels.

Longer barrels provide higher velocities, but are more prone to barrel whip and thermal induced shift in POI.

Thicker barrels don't heat up as quickly, and are more rigid, providing better consistency.

If money is no object on the gun and I'm shooting commercial 55gr bullets, I'd go for a 20in heavy bull barrel, non-stainless, non chromed, cut rifled barrel with a 1/9 twist.

And don't overlook the use of match ammo for maximum consistency.

[nvisn]

Thanks for the information all!

[Mopar]

My boss is looking to build a 600 yard AR-15 using commercially available 55-62 grain ammo. What barrel length and twist would be ideal for this type of application. Thanks for any input.

I'm not sure 55gr would even make it out to 600yrds with enough power to punch a hole in paper. Lets look at a decent 55gr ammo, Federal Premium - 55gr Sierra Gameking BTHP 3240fps. By 300yrds, half your distance, energy is already down to 465 ft lbs, and you have almost 11 inches of bullet drop, and in a 10mph cross-breeze you will have 14 inches of drift.

Most long range .223 shooters are using bullets at LEAST 69gr, and mostly using 75-80gr. Lets look at Federal Gold Medal match - 5.56x45mm NATO (.223) 77gr 2750fps. At 300yrds you actually have 38 inches of bullet RISE; at 600 yards you end up with zero rise or drop. Point of aim. At 300yrds you have 715 ft lbs of muzzle energy, and 9 inches of drift in 10mph crosswind. At 600yrds you still have 370 ft lbs of energy and drift is 43 inches. Even heavier would be better. Of course, to stabilize a 69gr bullet you need a 1:8 twist barrel (like I have on my national match rifle) and for 75-80gr bullets you really should go even faster with a 1:7 twist. Problem there is 1:7 will spin a 55gr bullet apart.

[edit to fix stoopid tiepoes]

[Mopar]

BTW, the closest 600yrd range I know of is Reading Rifle & Revolver club up in the North East corner of Massachusetts.

[MadSmith]

55gr will easily make it to 600y with plenty of ability to penetrate paper. But depending on innumerable factors, it would have to be a fairly large piece of paper.

Generally, I agree with the recommendations to use a heavier bullet, and a correspondingly tight rifling twist to stabilize it in order to gain the most effectiveness at long range.

A 62gr bullet (M855) should be able to penetrate 1/8 steel plate out to 600yd.

If you really want to stick to 55gr, 600yd is doable, but tougher, as the lighter bullet doesn't hold its velocity as well, and is more prone to crosswind effects.

Typically, you zero 55gr projectiles to 250yd. That puts POI less than 4in high at 100yd, and less than 4in low at 300yd. Good for nailing a man-sized target in a critical region pretty much anywhere within that range by aiming at his mouth. Or a more guaranteed hit at center mass by aiming at his solar plexus.