Will rifle length gas system in an 18" AR15 be OK?

[68bthp]

I'm going to have a krieger "Varmatch" barrel made in 18" instead of the usual 20" length. Krieger doesn't normally cut barrels shorter then 20" and will not guarantee reliable functioning. The rifle will have an LMT SOPMOD collapsible stock with a rifle-length gas system.I intend to shoot 68bthp or 77SMKs in this 1- 7.7 twist barrel. Will it function reliably?

Also; Can you tell me if I should go with a .093 or .097 gas port? .093" is their standard size for a 20" barrel....but they offer the .097 and I'm not sure if it will function more reliably in an 18" rifle length system.

 

[308MAUS3R]

Re: Will rifle length gas system in an 18" AR15 be OK?

18" barrel with a rifle gas system is no problem. I have one but I am not sure of the gas port size.

Are you going to run a mix of ammo or are you going to run a single load? If you are going to run a single load, I would get the larger gas port and an adjustable gas block.

 

[Redmanss]

Re: Will rifle length gas system in an 18" AR15 be OK?

Yes, it will work just fine. Just look at the MK12.

Gas port dimensions I defer to the experts.

 

[M21guy]

Re: Will rifle length gas system in an 18" AR15 be OK?

I'd go with the bigger port since you are cutting down the barrel 2" for a little extra reliability. Suppressed I suppose the std would be all that is needed, probably all that is needed in the first place, but to be safe I'd do the .097" port. Then it is there. You could tailor it with an adjustable gas block or maybe a bump in weight buffer if you didn't find it to your exact liking.

 

[Outsydlooknin75]

Re: Will rifle length gas system in an 18" AR15 be OK?

Personally Id see if they will do a midlength gas. Noveske 18 inchers are mainly mids for reliability. I read that with a rifle length and 18 with normaly gas hole they can get slow and not always function in cold weather.

If nothing else go with bigger, its better.

 

[68bthp]

Re: Will rifle length gas system in an 18" AR15 be OK?

Thanks gentlemen for the help thus far.

I want to keep it a rifle length gas system because I already have a $400 rifle length Daniel Defense free float rail that I'd like to use again...and gas tube too.

I intend to shoot 68-77 grain BTHP handloads. They will be loaded near max or whatever maintains tight 5/8" MOA groups that I'm attempting.

As far as a gas block, I'm not sure if they are available in .937" with an adjustable feature. But I'm thinking I'd like to use the DPMS A2 FSB anyway, as it's just my personal taste that a fixed FSB keeps an AR looking closer to its military roots. So, I'm trying to be careful and choose the right sized gas port.

 

[benchmstr]

Re: Will rifle length gas system in an 18" AR15 be OK?

never had my mk12 take a shit on me...so i guess it works fine

 

[223Rem]

Re: Will rifle length gas system in an 18" AR15 be OK?

Gastubes and low profile gasblocks are cheap.

 

[308MAUS3R]

Re: Will rifle length gas system in an 18" AR15 be OK?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Outsy</div><div class="ubbcode-body">Personally Id see if they will do a midlength gas. Noveske 18 inchers are mainly mids for reliability. </div></div>

Actually, on 18" barrels, Noveske uses "intermediate" gas systems which are between mid-length and rifle length and use a custom gas tube.

 

[showlow50]

Re: Will rifle length gas system in an 18" AR15 be OK?

I have 2 18" barreled ar rifles one 223 and one 308 with rifle length gas tubes. Both run great with light and heavey bullets.

 

[TonyAngel]

Re: Will rifle length gas system in an 18" AR15 be OK?

One of the advantages of going with a rifle length gas system on an 18" barrel is that it shoots so softly. Going with the larger port, without needing it, is going to detract from the advantage. I'd go with the smaller sized port. If it turns out that you need more gas, punching it out isn't a big deal at all.

When I was shooting an 18" rig with a fast twist, it was because I wanted to reach out with it, so I was running my loads on the hotter side for the velocity. With hotter loads burning a medium burn rate powder, I don't think you'll need the larger port.

 

[MitchInOR]

Re: Will rifle length gas system in an 18" AR15 be OK?

Compass Lake Engineering will turn you an 18 inch 7.7 twist Krieger barrel with a rifle length gas port and it will run just fine.

 

[68bthp]

Re: Will rifle length gas system in an 18" AR15 be OK?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: TonyAngel</div><div class="ubbcode-body">One of the advantages of going with a rifle length gas system on an 18" barrel is that it shoots so softly. Going with the larger port, without needing it, is going to detract from the advantage. I'd go with the smaller sized port. If it turns out that you need more gas, punching it out isn't a big deal at all.

When I was shooting an 18" rig with a fast twist, it was because I wanted to reach out with it, so I was running my loads on the hotter side for the velocity. With hotter loads burning a medium burn rate powder, I don't think you'll need the larger port. </div></div>

Tony,

I spoke to JP Rifles this morning. They use a .104 gas port on their 18" rifle length barrels. I also found an old thread on another websight in which somebody says that John at WOA uses a .108" port on his 18" SPR barrels. And still another gentleman on that same thread says that he has had a number of barrels shortened to 18" and 16" by ADCO without modification to the .093 gas port. He says the 18" always works well without issues, (while the 16" does not always) and he believes that the buiders only use the larger port for when these 18" barrels will be used in cold weather and/or with lighter loads. So you are right, and it seems there is some flexibility here when choosing a port size. To me it makes more sense to go with the smaller size port being concerned with tighter groups, because the less upset that happens to the bullet jacket riding over a smaller hole, the better. I'll probably ask for a .097" port, as that is the one of two listed sizes that Krieger offers. And others agree with you to say that the rifle length gas system will be a softer shooting rifle with less wear and tear.

Thanks for the help.

 

[Scottmilk9]

Re: Will rifle length gas system in an 18" AR15 be OK?

My humble opinion is that 18 inch barrels should be left at mid-length, but with that said, I have plenty of customers who like the rifle length and I have built many. Gas port size is tricky with such a variety of ammo, velocities and pressures are all different from handloads to each manufacturer of factory ammo. With these varietys of ammos, there is no definite answer how big the gas port should be. Most of mine usually are at .108.

On another note, what John at WOA says is like the bible, he knows his $hit!!