differences between mid length and carbine gas systems?

[breakingtime91]

Hey all, in the next year when I'm no longer stationed in California, I want to get a AR platform. I'm mainly looking at the LaRue Predatar and a DD V4 carbine length. What are the main differences of the gas systems (mid length vs carbine), if there are any? I carried a M4 in Afghanistan and never had a issue with the gas system, so just curious if the extra length is worth it.




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[LittleLebowski]

Slightly softer recoil impulse, longer gas tube. There is lots of theoretical benefits folks list but I just listed what I notice. Depends on how much money you have to spend as the Larue is very nice but a few hundred more than the DD. You really can't go wrong with either.

Are you on Pendleton?

[breakingtime91]

yes I am

[orionz06]

I would buy a mid length if given the option as the theory supports it being OK but it is not worth paying more or going out of your way to get it. I would not sell any of my carbine length guns to acquire mid lengths either, they all work if kept within the bounds of their design. The DD and LaRue are vastly different guns though.

[LittleLebowski]

Which camp? I ran the pools at Flores and margarita as a MCWIS for a while a few years ago.

[breakingtime91]

ya that's what I thought about the mid length systems, what are the differences between the two designs?

[SecondsCount]

The design is basically the same between a carbine, midlength, and rifle gas system. The main difference is the length of the gas tube.

The original design was the rifle length system with a 20" barrel and then later it was shortened for the 14.5" barrel when the M4 was designed. Civilians cannot own a barrel length shorter than 16" without jumping through some extra hoops so the civilian versions have 16" barrels. This increases the amount of gas that the bolt sees during the firing operation.

The midlength design moves the gas port out a little further on a 16" barrel which in theory should lower the gas pressure and balance things out.

I personally like the midlength system better but a quality built carbine system will run just as well.

[Al T.]

Biggest difference I see is that a midlength gives me more room to extend my left hand and is more comfortable. When I go back to my carbine length, it's noticeable.

[orionz06]

Biggest difference I see is that a midlength gives me more room to extend my left hand and is more comfortable. When I go back to my carbine length, it's noticeable.

Depending on the end configuration the gas tube length is a non-issue in terms of support hand position. Just get the right rail.



Back to the OP, to elaborate on my original statement: Let the desired end configuration dictate gas tube length. Many people really love the 12" rail with a FSP cutout, this requires a carbine length. I prefer a rail mounted front sight, this allows any gas tube length.

[breakingtime91]

I was leaning towards the Larue mostly because of their outstanding reputation but DD isn't bad either. I don't like owning a lot of guns so I usually own 1 for the roll it'll play. Seems like I have a hard decision to make.

[LittleLebowski]

I was leaning towards the Larue mostly because of their outstanding reputation but DD isn't bad either. I don't like owning a lot of guns so I usually own 1 for the roll it'll play. Seems like I have a hard decision to make.

The Larue is elegant, well made, and I want one. Were I you, I'd build up a DD or BCM exactly the way I want it and spend the savings on an Aimpoint Pro optic.

Do me a favor and get a carne at Colima's for me.

[JMS]

The design is basically the same between a carbine, midlength, and rifle gas system. The main difference is the length of the gas tube.

Yep. Carbine = 7" gas tube, mid-length = 9" tube, rifle = a little over 12" tube.

I've one of each, carbine and mid, and I'm not sensitive enough to note any palpable difference in perceived recoil feeding the same ammo through it, but that's got a lot more to do with "It's a 5.56 gun, how much recoil is there, really...?" than anything else.

If the plan is to feed it as much cheap .223 ammo as you can get your hands on (PMC, UMC, the steel-case offerings), sticking with a carbine-length system might be a better idea, since the mid-lengths do better with full-pressure ammo. BCM even has a disclaimer to that effect; having said that, I have one of their mid-lengths, and it has, to date, eaten everything I've put in it. I have, however, seen WAY more mid-lengths than I own, taking and teaching classes, so while it's not an absolute for any individual gun, I'd call it a valid trend among the mid-length population, as a whole, that they're better served being fed with ammo loaded to 5.56 pressures (chamber-pressure of 55k psi is the baseline; a proof load [what they use to test bolts, per the TDP] is 72k psi).

I'm with LL, RE: what to get first. I went full-retard on my first personally-owned gun; built a lower, got a Noveske upper to put on it. While I can't say that I regret the decision, it would have made a helluva lot more sense to take that $1250 (upper!) and +/- $300 (lower), and just gotten a Colt/DD/BCM carbine and an optic. In the same vein, you probably won't regret purchasing the LaRue, but you might love it while wistfully wondering how much ammo or which training evolution you could have attended because you sunk all the $$$ into the gun, alone. Just food for thought.

We were shooting vmi-mo's 7.62 OBR yesterday (I can only hope he and F2S didn't see my tumescence....). I wholly understand the allure of the PredatAR....

[EMC]

Another thing to note is the concept of "dwell time" which is the length of time the bullet takes after passing the gas block to exit the muzzle. On a 16 inch barreled carbine length gas system, the extra dwell time causes gas pressure to spike for a longer duration. This causes more violent extraction as the case remains obturated (sealed by expansion) in the chamber longer while the extractor is tugging on the case rim. I've experienced bent case rims and ejector "smearing" on higher pressure loads on my commercial 16 inch carbine before. This is less of an issue with higher quality TDP following manufacturers that keep their gas ports to the correct spec. Also less issue with 14.5 inch barrels on carbine gas as that allows for the same dwell time as rifle gas systems. It is another argument for going with midlength gas on a 16 inch barrel.

[SecondsCount]

....

If the plan is to feed it as much cheap .223 ammo as you can get your hands on (PMC, UMC, the steel-case offerings), sticking with a carbine-length system might be a better idea, since the mid-lengths do better with full-pressure ammo. BCM even has a disclaimer to that effect; having said that, I have one of their mid-lengths, and it has, to date, eaten everything I've put in it. I have, however, seen WAY more mid-lengths than I own, taking and teaching classes, so while it's not an absolute for any individual gun, I'd call it a valid trend among the mid-length population, as a whole, that they're better served being fed with ammo loaded to 5.56 pressures (chamber-pressure of 55k psi is the baseline; a proof load [what they use to test bolts, per the TDP] is 72k psi).
....

Valid point on the low pressure ammo. I have not tried Wolf in my BCM midlength but Tula and some of the other steel cased stuff will not run 100%. Mine has a standard carbine buffer with an auto bolt carrier.

It doesn't bother me as I run my personal reloads through the gun 100% of the time. I had lent my BCM to a guy in a class and he had the Tula ammo.

[breakingtime91]

I see what your saying about the difference in money and I probably will go with the DD and put the extra $ toward a Aim point mini