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View Full Version : AR barrel length.


mark501w
06-11-2013, 6:59 AM
How far past the gas port does the projectile have to travel for reliable functioning?

FeuerFrei
06-11-2013, 7:24 AM
16' barrel with port location diameter = .750 gas port is approx 8.375 from the muzzle.
YMMV
Gas port size is critical too.

CaliB&R
06-11-2013, 7:33 AM
Not counting pistol length, its usually around 6-8".
On a 14.5" barrel with a carbine length gas system, the port is ~7.8" from breech or ~6.7" from crown
On a 16" barrel with a carbine length gas system, the port is ~7.8" from breech or ~8.2" from crown
On a 16" barrel with a mid-length gas system, the port is ~9.8" from breech or ~6.2" from crown
On a 18" barrel with a mid-length gas system, the port is ~9.8" from breech or ~8.2" from crown
On a 20" barrel with a rifle length gas system, the port is ~13.17" from breech or ~6.8" from crown

Plus FeuerFrei is correct, port size is important as well. Shorter gas lengths need larger ports. I have a mid-length on my 16" barrel because its a smoother recoil than carbine length.

mark501w
06-11-2013, 8:07 AM
OK , what about these Sp1 & A2,A3 style guns with their 16" barrels? What is the inside diameter of the gas tube?

FeuerFrei
06-11-2013, 9:52 AM
.125 if I remember correctly. Measure with pin gauges or drill bits to be sure.
Is there a point to your Q&A? Literally all AR tech specs are available online somewhere so this info is readily available.
Are you doing some experimentation on a rifle?

mark501w
06-11-2013, 10:27 AM
I was curious as to if these guys that are building these full length rifles are with loped off barrels are having functioning problems? I'am not a fan of the M4.

FMJBT
06-11-2013, 11:05 AM
I've noticed the smoothest recoil impulse on 18" barrels with rifle length gas system. 14.5" mid length gas system is pretty smooth too. IMHO, the 16" mid length is still a bit over gassed, and should be run with an adjustable block. I wish more 16" barrels were offered with an intermediate length gas system. 16" with rifle gas can be made to run, but the port size needs to be opened up to nearly 1/8", and even then needs to be run with full power NATO spec ammo in most cases to be somewhat reliable.

russ69
06-11-2013, 11:07 AM
I was curious as to if these guys that are building these full length rifles are with loped off barrels are having functioning problems? I'am not a fan of the M4.

I think that once the bullet passes the gas port, it's job is done.

FMJBT
06-11-2013, 11:11 AM
I think that once the bullet passes the gas port, it's job is done.


That's actually when the bullet starts doing it's job, as far as cycling the rifle goes anyway.

mark501w
06-11-2013, 12:28 PM
Just curious thax.

ar15barrels
06-13-2013, 12:22 AM
SOAR79's wife said he is not even packing 5", but I make her happy...

SOAR79
06-13-2013, 12:42 AM
^^I've heard you need at least 5 inches

SMarquez
06-13-2013, 8:05 AM
^^I've heard you need at least 5 inches
That's what she said.....

joeblowphony
06-13-2013, 8:33 AM
That's what she said.....

Just 5? She didn't want to hurt your feelings.