Question on longer barrels

[Nuclear_Arms]

I've been mulling around the idea of a more precision oriented upper using an 18" or 20" barrel. I currently have two 16" complete ARs with A5 kits and Geissele triggers. One of them uses a Centurion CHF light weight, and it has performed well for me, but I'm curious about going with a full on stainless match barrel.

I have been doing lots and lots of reading, and have come up against a conundrum of sorts. The 18" rifle gassed upper (a la the SPR) has a lot of proponents, but guys who I trust as SMEs talked a lot about temperature sensitivity and other reliability issues. However, these posts were written back in 2006/2007, and I haven't seen much written about it since. Have the kinks of the gas system been worked out in the latest versions? I would probably be going with a BCM SS410 based upper, or the Centurion DMR.

There is also a bit of appeal to the 20" upper as well, in a kind of M16A4-ish way, but the options seem more and more limited (the BCM SAM-R barrels notwithstanding).

Is there going to be any significant difference in the ruggedness and reliability in these upper patterns? I see a lot of 3-gun guys run the 18" pattern, and the primary use for this upper would be in competition and marksmanship practice and longer ranges (400-600 yards). Or is the only difference going to be in weight?

Or, I could just forget the whole thing and stick bigger glass on my current 16" Centurion upper.

[Stickman]

I have a Centurion Arms DMR upper, and have zero complaints with it. I'm not sure where the temp issues are coming from as these barrels are being used up in the mountains of Afghanistan in cold weather, and I don't hear complaints. Ammo issues may be part of it, but without knowing who you personally spoke to and what their actual issues were, it is just a guess.

[Nuclear_Arms]

Thanks, Stick.

It was a thread that popped up about Mk12s in the M4C tech forum. I kept digging around and found as much as I could. KevinB, DocGKR, K.L. Davis, Grant and a few others were commenting that the 18" rifle gas got finicky in cold weather (I would imagine with certain ammo loads). The opinion of those guys was that the 20" barrel was just all-around better. But, as I mentioned, these threads were all about 7 years old. They mentioned that a lot of builders were going away from 18" rifle barrels to 18" middy (or intermediate) barrels. Yet, seven years later, it seems that the 18" rifle gas is the most common.

I was just curious. I would rather save the weight on the end of the gun. One of Monty's Mk12 barrels (or Paul's SS410) mounted with a Fortis REV sounds mighty appealing a the moment.

[RPM509]

If I may chime in. Hazarding a guess, this may be an issue of combining barrel OAL, gas port location and ammunition; not just an 18" barrel vice a 20" barrel.
With a rifle length gas port on an 18" OAL barrel, the dwell time would be two inches less than the same gas port location (rifle length) on a comparable 20" barrel.
One type of ammunition that may cycle flawlessly with the 20" may not function in an 18" given all else as equal. This may also point to other issues such as gas
port hole diameter, buffer weight, spring etc not being ideal for that particular barrel length and other parts in combination.

[Nuclear_Arms]

That's about what I figured. At this point, I don't think it would hurt anything to try the 18" rifle gas system mounted to an A5 kit. Only reason I can think of to go to the 20" would just be for the little bit of extra weight for even more recoil absorption.