Mid vs. rifle on 18" .308

[thermobollocks]

Those of you who have experience building .308 AR-10s, hopefully parts kit builds, what are the pros and cons of the two prevalent gas options?

Impacts in using full vs. reduced mass and adjustable vs. fixed gas setups in each?

[Lifeislarge]

Watching this as I just picked up a matched stripped upper/lower receiver set this weekend for my first AR-10 build. So far I've learned that I need the Armalite "A" lower parts kit and the DPMS thread barrel nut. Who knew?

[kurtm]

308,s in general are WAY over gassed! I personally would never build one without an adjustable gas block. Always go rifle length if it is an option. This all applies for full weight or light weight components.

[Brian04rs]

I read the other day rifle length gas will keep the bolt closed for longer which is good from a accuracy standpoint.

[kurtm]

De-burring a flash hole in a case will give far better accuracy gains than the micro millisecond difference between when a mid length or rifle unlocks! Bordering on urban legend in my view.

[Brian04rs]

Just something I read. Isn't everything on the internet true?

[thermobollocks]

308,s in general are WAY over gassed! I personally would never build one without an adjustable gas block. Always go rifle length if it is an option. This all applies for full weight or light weight components.

That's what I remember from Internet and 3-gun match lore (which is what we're all doing anyway, right?)

Why, then, is there such an abundance of mid (and even carbine!) gassed barrels? Because most people who buy them are only going to burn 4 mags once a month and it has to work with Tula?

Edit for English.

Edited May 6, 2016 by thermobollocks

[TonytheTiger]

308,s in general are WAY over gassed! I personally would never build one without an adjustable gas block. Always go rifle length if it is an option. This all applies for full weight or light weight components.

That's what remember from Internet and 3-gun match lore (which is what we're all doing anyway, right?)

Why, then, is there such an abundance of mid (and even carbine!) gassed barrels? Because most people who buy them are only going to burn 4 mags once a month and it has to work with Tula?

Because the military uses a carbine length system, automatically making it the best there is. Oh and I know the people who buy them, they're only burning two mags every four months, and sometimes it's Wolf instead of Tula.

[HRider]

I spoke at length with John Paul a few years ago on this very subject at the Pro Am. He builds Mid length systems for 18" barrels and doesn't offer a 20" light contour barrel with a rifle length gas system (what I want). He said that mid length works over a broad spectrum of barrel lengths and is tunable with adjustable gas. He said that I would not gain anything noticeable with a 20" rifle gas over an 18" mid length. I explained that I am an irons shooter and wanted the barrel length for sight radius. He finally told me that he could do a custom barrel for me, but really didn't want to. I have shot a few mid length systems with different comps (none with a Titan, though). My 20" rifle length, non adjustable gas Armalite with a Titan is the softest one that I have shot so far.

Hurley

[rboyes]

Not much difference in the two lengths. Rifle gas port should see less erosion compared to midlength. Adjustable gas can make them feel the same. I'm currently using a 16" rifle length gas barrel in my .308. Works great.

[co-exprs]

Mine is the same experience. I've run rifle length 18 and mid length 18. Both felt over gassed to me and needed an adjustable gas block to fine tune the performance. I have to agree with JP. 308's experience more variance in pressure and gas volume than 223. Once the 18" mid gas block was set, it worked great with most anything I fed it. The 18" with rifle length gas has less dwell, making it a little more finicky. Not a lot more, a little more. This would manifest as some an occasional failure, if I set it up right on the edge, using my 155gr load (410pf), and then ran my softer 125gr ammo (375pf), or some nato ammo (365ish pf).

If your goal is to run without an adjustable gas block, then an 18" rifle length gas will likely serve you well. If your goal is to have the softest, smoothest 308 action, then adjustable gas is important and the gas length won't matter as much.

Edited May 6, 2016 by co-exprs

[gunther]

I seem to have a oddball rifle, its a cmmg mk3 cbr with a rifle length gas system 16" long barrel. It seems to run smooth to me.

[rboyes]

Powder selection also makes a difference in your rifle's operation. I've found that slower powders in my 16" rifle gas gives a wider operational window.

[co-exprs]

True on the powder selection, but now you are using powder burn rate to manage operating pressure. Slower powders such as Varget are not optimum for shorter barrels. So now you are giving up velocity to gain operating pressure just for the sake of running a rifle length 16" barrel. Why? By running a mid speed powder like H4895, you get a more complimentary burn rate in your short barrel semi auto and a mid or intermediate length gas system is a better match for the pressure curve and the additional dwell makes it more forgiving over a wider range of loads.

[rboyes]

But you get better fireballs with slow powder and short barrels! J/k I use IMR 8208 for .308.

[mic2377]

To be honest, whether in 223 or 308, I have preferred the longest possible gas system when it comes to recoil impulse.

The big problem with many 308 is being overgassed, and the shorter dwell time reduces that. The longer gas system also seems to have more latitude for running reliability if suppressed. This is really more important for those with a fixed gas block.

I have a Mega 18" barrel with mid-length gas. It was HORRIBLY overgassed and would not make through even 5 rounds without a malfunction, and sharp recoil impulse. I got it running reliably but it has been very touchy with gas setting. The Criterion rifle gas barrels I have used were the opposite - smooth recoil and 100% reliability with any ammo.

However I can certainly see that if running weaker loads, or lighter bullets with faster powders, that the shorter gas may have a reliability benefit. In other calibers, IE 300 BLK, shorter gas length is a huge benefit for reliability. I only have experience with a few barrels, not the larger sample size of others here.

[Snopczynski]

308,s in general are WAY over gassed! I personally would never build one without an adjustable gas block. Always go rifle length if it is an option. This all applies for full weight or light weight components.

^^^^^^ THIS! My AR10 is on the first click of the Gas Block setting. Go Rifle Length for sure.

[RayUSPSA81]

It would depend on your applications and platform. I would go 16 on an AR and 18 on a M1 I love the scout sized model.

[DocMedic]

The DPMS and GAP-10 are both Mid-length even though the GAP-10 is a 18" barrel. I'm thinking I might rebarrelled the DPMS with a 18" barrel with a adjustable gas block to see what accuracy I can get out of it as I'm getting tired of lugging the GAP-10 around on my back at 3gun stages (15lbs rifle...) The GAP-10 does not have a adjustable gas block but has a very efficient surefire comp that was tuned by GA precision. The rifle has virtually zero kick, but does suffer from lower PF ammo stoppages, as I found out at He-Man this year.

These are just my experiences.

[thermobollocks]

Was it the low PF or was it other stuff going on? Do they drill the gas hole small on the GAP? My goal is basically minute-ish accuracy with the crap that comes off my Dillon, around 10# including glass, and not going to shear the bolt lugs inside 5k rounds/gum up inside of 500 rounds for reliability.

[Poppin]

I am currently in the final stages of my first .308 build. I opted for a light weight DI gun as I have 3 piston POF's and was wanting to round out my AR stable with a DI in .308. Looking for a lightweight unit and have lrocured the following parts. Aero SOCOM blue builder set with 15" Atlas "S" handguard, 18" BSF fluted CF barrell (rifle length gas sys), Superlative arms adj gasblock, RCA full titanium BCG and a captured one piece buffer assembly such as a JP Enterprises or Armaspec. I have used all of these parts in my last build but it was a pinned and welded .223 Wylde that weighd in a tic over 5#. It also had a Brigand Arms C/F handguard. It is a blast to shoot. Just wondering if anyone can see a problem with my chosen components as my experience with them are very favorable but all dealing with 5.56 AR 15's. I am hoping they all function as well in the AR10 platform. Any advice in much appreciated. Merry Christmas to everyone!!

[Poppin]

I am currently in the final stages of my first .308 build. I opted for a light weight DI gun as I have 3 piston POF's and was wanting to round out my AR stable with a DI in .308. Looking for a lightweight unit and have lrocured the following parts. Aero SOCOM blue builder set with 15" Atlas "S" handguard, 18" BSF fluted CF barrell (rifle length gas sys), Superlative arms adj gasblock, RCA full titanium BCG and a captured one piece buffer assembly such as a JP Enterprises or Armaspec. I have used all of these parts in my last build but it was a pinned and welded .223 Wylde that weighd in a tic over 5#. It also had a Brigand Arms C/F handguard. It is a blast to shoot. Just wondering if anyone can see a problem with my chosen components as my experience with them are very favorable but all dealing with 5.56 AR 15's. I am hoping they all function as well in the AR10 platform. Any advice in much appreciated. Merry Christmas to everyone!!