18" Barrel Gas System Thoughts

[joebush]

Hello,

This is my first post here. I have been reading some articles about avg. pressure and dwell time with the various length gas systems. Have you had any issues with the 18" barrels using rifle length gas systems? Nothing exotic here just standard 5.56, no suppressors, etc.

Joe

[letaz77]

Hi Joe,

It is good to have you here o the forum. I happen to be the lucky owner of an 18" barrel with rifle length system and have never had any issues at all. My rifle cycles flawlessly with all kinds of ammo, from cheap production or surplus to high-end match ammo and hand loaded ammo.

I don't think you could go wrong with that set up (18" rifle length).

Erick

[brodezoe]

Hey Joe and Erick,

I'm afraid I don't have anything to add to Joe's question, but I'm keeping an eye on this thread because I've been wondering about the correlation between barrel length and cycling. I'm thinking of getting myself a 20" (and have been shooting the standard M16 20" for years now) and am very interested to see what unfolds here.

brodezoe

[joebush]

From what I have been able to gather over the last few weeks, barrel length and gas system length correlate this way:

I have tried to pull more information out of it, but can't seem to wrap my head around it. Per ar15barrels.com the ideal dwell time is .200 ms, which is very close to the dwell time of both 20 inch rifle systems and 14.5 inch carbine systems. I hope my wierdo setup of 18 inch barrel and rifle length gas system works out alright. I am hoping it puts the least stress on the action and still cycles reliably. We will see.

[wolfgangmob]

Absolute worst case you'll need a muzzle device that increases back pressure. That said, people run rifle length systems on 17" barrel from Noveske so an 18" should work well, just may not want to run too light of a bullet which would decrease dwell time, but if you're getting an 18" with 1:8 you are probably sticking to heavier weights, also 28gr of 748 behind a 55gr bullet is going to be clocking around 3100-3200fps or higher, if you use heavier bullets and get lower velocity you should have higher dwell times, if you use hotter loads you'll have higher pressures to make dwell time less important. Keep in mind that the military uses rifle length on an 18" on the Mk 12 SPR.

[rwf]

wow, some of you have put a lot of thought into this. just my 2 cents, but unless you have cut the barrel down, (made it shorter) it should be set up already with the correct size gas port for the set up you are running.
but with that said, i always use a adjustable gas block set up to just above failure. but that's the 3 gunner in me though.

even did that with my F-class upper but for a different reason, had extra wieghts in the buffer, heavy carrier and set up with a suppresser and it would not function with 223, just 5.56 to keep it locked for the longest amount of time.

just the opposite of the rest of my AR's

[bravo5two]

I've been running an 18" WOA SPR barrel 1:8 with rifle length gas system using 55 gr bullets for the last 3 years without any issues. It has been tuned to run a low mass operating system from JP and the adjustable gas is about 50% open with "hosing" loads 23.5 gr of H335 (2700 fps), soft and easy on the barrel when encountered the 40-50 rds hosing stages.

[rwf]

my kind of shooter!

[greentick]

Interesting chart above. I have been using a 16" with a rifle length gas system for 15yrs, just has and A2 FH. No probs from 52 to 70gr bullets.