11 inch barrel gas length?

[midnitesi]

Im looking into swapping out the 300 blackout barrel on my mk18 mod1 to an 11 inch 6 arc barrel. The couple sites im looking at allow you to choose the gas tube length for the barrel. Options are pistol, carbine and midlength. 

my concern comes from way more experience with 556 than 6 arc. I know i need to account for dwell time. My goal is to keep my mk18 clone but upgrade to a better round. It will be suppressed with a sandman k. 

can someone with way more arc experience school me on tube length for 6 arc barrels? Id love to go mid length, but i know mid on 556 requires tuning. I want to avoid adjustable gas blocks, lighter springs or buffers, etc. 

current set up for consideration. 
geissele braided H1 buffer and braided spring, SSA-E trigger, PSA grendal BCG, geissele CH, deadair brake, sandman k. 
running either 108 ELD-Ms or 77 grain cutting edge bullets. 

all advice is appreciated.

[grayfox]

I don't have a lot of experience ( a little with 556 and 300 blk) in shorter barrels, but would think your best choice would be carbine length. Just 2 cents however.

Look at how much barrel lies beyond the gas block to the muzzle, that's your dwell-time "length". 6Arc pressures being Saami limited to 52 ksi, vice the 556 or 300 Blk at (IIRC) 56 ksi, inch for inch the 6Arc should have a lower pressure than either; and gas volume right between those 2 via the bore size. So I would vote for the carbine length. I personally don't prefer shorter gas tube lengths b/c that means more back pressure to the bolt... and possibly into my face, leakages from the chamber area I mean.

[midnitesi]

I don't have a lot of experience ( a little with 556 and 300 blk) in shorter barrels, but would think your best choice would be carbine length.  Just 2 cents however. 

Look at how much barrel lies beyond the gas block to the muzzle, that's your dwell-time "length".  6Arc pressures being Saami limited to 52 ksi, vice the 556 or 300 Blk at (IIRC) 56 ksi, inch for inch the 6Arc should have a lower pressure than either; and gas volume right between those 2 via the bore size.  So I would vote for the carbine length.  I personally don't prefer shorter gas tube lengths b/c that means more back pressure to the bolt... and possibly  into my face, leakages from the chamber area I mean.

that is my normal way of thinking for dwell times etc. the kicker is im seeing 18, and 20 inch barrels with rifle +1 or +2 length gas tubes, and still running just fine, specially the 18 inch rifle +2 ones. which has me wondering if dwell times is not really much of an issue with 6arc.

[grayfox]

I think that had something to do with the mil spec DOD gave to Cmmg and maybe proof, they are about the only ones doing that. Not too many others are doing it, they are using more typical gas lengths.
I was thinking you could maybe run some test cases with Grt but it does not take into account any gas block locations. Real problem is, if you order one with the gas length too long and it doesn't work, you have a $$$ tomato stake.

Come to think of it, look at BFT's website, sixfivearms.com, He has both grendel and 6Arc 12" barrels, he usually does a good bit of research, and both of those are CLGS. Send him a pm from on here and ask if you want to.

[midnitesi]

I think that had something to do with the mil spec DOD gave to Cmmg and maybe proof, they are about the only ones doing that.  Not too many others are doing it, they are using more typical gas lengths.
I was thinking you could maybe run some test cases with Grt but it does not take into account any gas block locations.  Real problem is, if you order one with the gas length too long and it doesn't work, you have a $$$ tomato stake.

Come to think of it, look at BFT's website, sixfivearms.com,  He has both grendel and 6Arc 12" barrels, he usually does a good bit of research, and both of those are CLGS.  Send him a pm from on here and ask if you want to.

I know i waited almost 2 years before i could get an 18 inch with rifle gas system, everything for a long long time was plus 1 or 2. i was going for a mk12 clone and those plus systems were no bueno. I have both of those sites up. is BFT the member name here? or are we able to tag him into this post? (been about 20 years since my last forum runs haha).

[grayfox]

O, sorry! I use a lot of abbreviations, easier to type! BFT stands for bluntforcetrauma. He is the admin, and the guy who created this forum in the first place. He's also the creator of the 65Grendel forum. And owns the sixfivearms website online store (soon to be called Rexus but the sixfive website always worked better for me... same thing but 2 names)
You should be able to PM him on either of these forums under his screen name bluntforcetrauma.

[SpeyRod]

My 12” Arc has a carbine length gas tube and it runs like a top. I would go with carbine.

[midnitesi]

My 12” Arc has a carbine length gas tube and it runs like a top. I would go with carbine.

I know those run great, Im more or less curious if the same rules apply to 6arc as they do with 556 and dwell time.

[SpeyRod]

My 12” Arc has a carbine length gas tube and it runs like a top. I would go with carbine.

I know those run great, Im more or less curious if the same rules apply to 6arc as they do with 556 and dwell time.

As always the answer will be it depends.

5.56 is running at 62,000 vs Arc at 52,000. The rules would be similar but not the same. Some differences that change the equation are case design, projectile weight, powder charge, gas port size, pressure and velocity.

Everything adds up to successful cycling of the action, or not.

I am running a +2 gas tube length on a 20” Bartlien barrel. I read about some people having issues with Varget in barrels with +2. In my case Varget loads run perfect from low to high charges. I have run mid length gas tubes on 5.56 guns down to 16”. Everything shorter I am running carbine or pistol length. But I am running cans on everything with one exception being a 18” competition gun.

On your 11” barrel you should be fine running a carbine length system that leaves @ 3 1/2” in front of the gas port. If you suppress it you may even need an adjustable gas block depending on your port size.

Good luck.