How Reliable are Carbine Length Gas System AR15s?

[M1Rifle30-06]

Whatever route you end up going, I suggest not replacing parts on any firearm until you have identified a definite function problem with that part. Don’t swap to a new telestock until you know what you don’t like about the original. Don’t add “reliability enhancements” that fix problems you haven’t had yet.

I’ve got about $300 of dubious “enhanced reliability” parts for various firearms sitting in my parts box right now, so I’m not just making this up.

I'll forgo the LMT "enchanced" BCG and whatnot, and just shoot whatever I get for now. Thanks for the advice.

[kymasabe]

Sig M400. Super reliable, carbine gas system. The new M400 Tread is affordable too.

[M1Rifle30-06]

Now, if you join SOCOM and shoot high round counts through a suppressor...get back to use then.

Does SOCOM not use carbine length gas systems anymore?

[davidsog]

Never had an issue with the carbine length in the M4 and M4A1's. Sure as heck more reliable than the M16A1 and M177's we had in 1/75th.

As for the suppressors.....

The one's in the SOPMOD kits hit the round count limits before the gas tube length became a problem but very few folks used the suppressors available in the kits. They have grown more popular since then.

[Metal god]

If your talking a 16" barrel then a middy is the way to go , 14.5" barrel and carbine is what I'd get . That's what I'd be looking for if I had one rifle and it needed to operate regardless . Keeping in mind the carbine gas system is designed for the 14.5" barrel . A 16" mid and a 14.5" carbine have pretty close to the same dwell time .

As Mobuck points out I to have custom gassed AR's that are the cat's meow to shoot in perfect conditions . I also have my carbine and middy's that are slightly over gassed and that's the way I want them .

I've turned a corner in my collection to where I feel there is not a need to have every firearm I buy now be a good SHTF weapon . So now I can play with trigger jobs on revolvers that may not be the most reliable with some ammo ( just don't use that ammo . Or gas systems that create the softest smoothest shooting AR you may ever shoot . Neither of those firearms would be the first I grab from the safe but that's OK I'm set for the Zombie's when they come because of others I have .

[M1Rifle30-06]

If your talking a 16" barrel then a middy is the way to go , 14.5" barrel and carbine is what I'd get . That's what I'd be looking for if I had one rifle and it needed to operate regardless . Keeping in mind the carbine gas system is designed for the 14.5" barrel . A 16" mid and a 14.5" carbine have pretty close to the same dwell time .

As Mobuck points out I to have custom gassed AR's that are the cat's meow to shoot in perfect conditions . I also have my carbine and middy's that are slightly over gassed and that's the way I want them .

I've turned a corner in my collection to where I feel there is not a need to have every firearm I buy now be a good SHTF weapon . So now I can play with trigger jobs on revolvers that may not be the most reliable with some ammo ( just don't use that ammo . Or gas systems that create the softest smoothest shooting AR you may ever shoot . Neither of those firearms would be the first I grab from the safe but that's OK I'm set for the Zombie's when they come because of others I have .

I used to think the same thing about 14.5" and carbine gas being optimized, but it appears the testing at Crane shows otherwise. The midlength 14.5" had far fewer stoppages. However, I still have some reservations about that test. Even the midlength 14.5" did worse overall than the Army's claimed 1,691 Mean Rounds Between Stoppages with the M4A1 using M855A1, and back in 2009 the Army claimed over 3,600 MRBS with M855 Green Tip. Something's not right.

[Metal god]

I used to think the same thing about 14.5" and carbine gas being optimized

Sorry didn't mean optimized just that a carbine length gas system is better designed for a 14.5" then a 16" barrel . Obviously if you start changing gas port sizes and other things the extra gases the additional dwell time creates in the 16" barrel can be mitigated and maybe that's what the civilian market does .

but it appears the testing at Crane shows otherwise. The midlength 14.5" had far fewer stoppages.

Interesting for sure , I guess there's always trade offs . The military seemed to think the 14.5" barrel and carbine gas system was best for there application . I've never dove deep into there reasoning .

[Bartholomew Roberts]

I understand the difficulty of making a decision when all you have is what you read as a base of knowledge. With all the information we have available, you can do pretty well like that; but you still need to get out and use them.

Just remember, it is an entire system. Mags, ammo, and operator error are the most common causes of failure. Make sure your budget includes room for good magazines, good ammo, and most importantly - good training.

All three of the rifles you are looking at (SPM-16, LE6920, and URGI) are used professionally by people who train with them and may have to use them in actual hostilities. All of them should provide a large buffer of reliability. All of them will likely require a five figure dollar amount of ammo before you need to start replacing parts.

I’d buy the one that fits your budget best. If you’ve got surplus cash, then the free-floated barrel and the longer sight radius of the midlength are nice. Longer handguards are also a plus when that barrel gets warm in training. None of that will make much difference at home defense distances though. And sight radius is irrelevant at any distance if you use an optic like most do.

The main thing is budget training first, then mags, then ammo, then accessories (cases, slings, etc.) then buy the rifle you want. You can read all the studies and NDIA powerpoints; but at the end of the day you have to shoot them to learn more.

[Nanuk]

I run a 16" mid length system in my Franken rifle. It runs from 100 above to -40. I had to tune the buffer and spring to the ammo, not the temperature.

[M1Rifle30-06]

I understand the difficulty of making a decision when all you have is what you read as a base of knowledge. With all the information we have available, you can do pretty well like that; but you still need to get out and use them.

Just remember, it is an entire system. Mags, ammo, and operator error are the most common causes of failure. Make sure your budget includes room for good magazines, good ammo, and most importantly - good training.

All three of the rifles you are looking at (SPM-16, LE6920, and URGI) are used professionally by people who train with them and may have to use them in actual hostilities. All of them should provide a large buffer of reliability. All of them will likely require a five figure dollar amount of ammo before you need to start replacing parts.

I’d buy the one that fits your budget best. If you’ve got surplus cash, then the free-floated barrel and the longer sight radius of the midlength are nice. Longer handguards are also a plus when that barrel gets warm in training. None of that will make much difference at home defense distances though. And sight radius is irrelevant at any distance if you use an optic like most do.

The main thing is budget training first, then mags, then ammo, then accessories (cases, slings, etc.) then buy the rifle you want. You can read all the studies and NDIA powerpoints; but at the end of the day you have to shoot them to learn more.

Alright, that helps me make the decision. Honestly, since all 3 of them have a large buffer for reliability, I think I'm going to go with the LE6920. I'll be able to buy a case of ammo, a sling, and several magazines with the money left over, and have something that can do more than I'll ever need it to do. I do like the proven track record of the Colt 6920 and the M4 that it's derived from. Even if the midlength does offer some nice advantages, the fact that the LE6920's select-fire brother has been fielded and used in combat by the thousands with good outcomes gives me confidence. Thanks for the advice.

[Metal god]

Good choice and don't look to much into what I'm about to say in regards to your choice , it's a good one .

the fact that the LE6920's select-fire brother has been fielded and used in combat by the thousands with good outcomes gives me confidence. Thanks for the advice.

Yes and no , The M4 the military uses has a 14.5" barrel and is not the same as the colt LE6920 with a 16.1" barrel . That extra 1.6" of dwell time changes the gas system . they may both have the same carbine gas system but they have different dwell times . Which in actuality means they have different gas systems .

Dwell time is the distance from the gas port on the barrel to the muzzle . This area is important because it's what creates the available gas/pressure that is redirected down the gas tube to cycle the action after the bullet passes the gas port but before it exits the barrel . The longer the dwell time the more gas is redirected down the gas tube .

Am I splitting hairs ? maybe a little but it is a difference and is why a mid length gas system is IMHO better on a 16" barrel .

Carbine gas system on a 14.5" barrel has 7.5" of dwell time

Mid length gas system on a 16" barrel has 7" of dwell time

Rifle length gas system on a 20" barrel has 8" of dwell time

There seems to be an area/length an AR gas system should be .

The civilian market needs to have a minimum of a 16" barrel so they first just added the extra 1.5" to the barrels . Carbine gas system on a 16" barrel has 9" dwell time which equals to much gas redirected to the action . To much meaning more then will ever be needed , not to much to operate .

I'm not pointing these things out to confuse you or to change your mind , you made a good choice . It's just to clarify some things .

As Mobuck and I pointed out earlier we both have 18" barrels with rifle length gas systems . That's only 6" of dwell time and depending on the gas port barely enough gas to cycle the action which results in an incredibly soft and smooth firing rifle . I also have a muzzle brake on my rifle . The speed in which I can follow up and get back on target with my 18" rifle is incredible . How ever as Mobuck pointed out we are on the edge-ish of malfunction and need everything to be going right to have 100% reliability do to the less gas being directed to the action because of only having 6" of dwell time and the longer gas tube .

[M1Rifle30-06]

So it sounds like you're saying... get the SOCOM with the 14.5" pinned barrel!

Just kidding. Though the SOCOM is tempting.