It looks like I'm going to be building an upper for myself soon. I'm looking to build a light weight upper for a calling gun so I'll probably go with a 16" light weight barrel. I notice I can get 16" barrels with either carbine or mid-length gas port holes pre-drilled. My question is is there an advantage to one over the other? If so what are they?

Mid-length gas blocks allow for a lower pressure gas impulse to work the action. With the lower pressure gas impulse, you get a slower and less violent rearward movement of the bolt carrier with smoother feeding and ejection.

The advantage of the carbine length is being able to run your bayonet! Doesn't everybody run a bayonet?

The downside is your bolt will crack probably twice as fast, but you'll have a bayonet to fall back on!

Ummm, your bayonet would be a bit stubby on a carbine gas system...

Like said above, it less pressure with a longer sight radious thrown in.

Downside is it's a fairly new system that while sort of proven has a bazillion rounds to go to even get close to the time proven carbine gas system in all weather conditions and ammunition.

I have a carbine, mid and intermediate, setup with the right gas port sizes and spring/buffers, I can't tell you one will outlast the other or is any easier to shoot.

That said, given a choice, I'll go midlength when I can but when I grab my Colt light weight, I know it's gonna work.

Yes, but that is why it is there....really.

Stubby? Yes, but the hoards in Haiti were sufficiently intimidated by "stubby" a time or two.

And Colt M4's routinely break/crack bolts...really. This ain't hearsay. This was extremely rare with the old 20in rifles but fairly common today.

M4's are also famous for the dreaded "bolt over base" malfunction that will ruin your day. I believe the high port pressure causes this.

Go with mid-length. If you're running A2 style sights it gives you a longer sight radius in addition to the already listed benefits outlined by MichiganScott.

The carbine gas system has been proven on 14.5" barrels aka the M4. The 16" barrel combined with the midlength gas system is ideal.

One last benefit, the carbine gas length looks really goofy with a 16" barrel when it comes to proportions. The mid-length looks a lot better.

So that's performance, reliability, and looks all improved. Could you ask for more?

my colt carbine seems to throw brass further than my Noveske midlength which piles it up neatly in the same area. I still prefer the carbine because I know its going to work until it breaks really bad. I would not want a carbine gas system on a second tier gun like a smith, or armalite, bushmaster, or dpms because you know up front they cut corners on their components.

Ummm, the USGI standrad issue M4 has a 14.5" barrel, the govt has never used a 16" barrel....really.

I wouldn't say "routinely" but yes they do break, however the service life is well above anything we need to worry about here, though I do carry a spare Colt bolt. Also you're talking 14.5" barrels not 16"s. Yes the extra 1.5" makes for more pressure on the bolt but after firing thousands upon thousands of rounds in a 16" carbine, I'm not skeered.

If the "bolt over base" is bolt bounce, that is why we have H and H2 buffers with loose weights inside, run a soldi buffer like the 9mm buffer and see what happens.

TWR, a 16in and a 14.5 M4 have the same gas port pressure, if the guns are other wise configured the same. The longer barrel has a slightly lower pressure at the crown because it has a longer dwell time.

A 16in barrel also pushes M855's effectiveness out about another 50-75 yd. By that I mean it'll break at the cannelure like M193. Mid-length 16in is the way to go.

Having carried an M4 on a few continents, I never knew how long the damn barrel was, thanks.

You are correct that often a buffer change can fix all manner of problems.

Bolt-over base is when a piece of brass is extracted but doesn't clear the ejection port and gets lodged ABOVE the bolt and under the charging handle. If you pull the charging handle, it jams it harder.

A Leatherman MUT is the fastest way to clear this, without a proper tool, just beat the butt against the ground for inertia to unwedge the bolt.

Yes, if you've ever seen a bayonet on a 16" carbine gas gun, you'd understand stubby. I'm pretty familiar with the 14.5 and 16" carbines gas systems but the longer barrel past the gas port actually increases pressure. That's why so many say the 14.5" with a mid length gas system is so smooth to shoot, less dwell time.

I've chronographed Colt 14.5", 16" and 20" barrels with m193 and found there's 106fps difference between the 16 and 20 and 101fps difference between the 16 and 14.5" (averaged of the 3 barrels I was testing). The 16" with a midlength system would be a better gun but it took Colt 15 years to convince the DOD to add a stiffer insert and extractor spring to the TDP, I'm afraid we'll never see a 16" M4 issued but as you pointed out, it would extend the range.

And thanks for explaining the bolt over base.

Thanks for the info. It was very helpful.

I'm going to bring this back up because I have another question -

I'm looking at stocks/buffer combos for this set up and I'm wondering what I need if I go with the mid-length gas system. I prefer the solid A2 style stocks, but I have no idea what I need for a buffer/spring if I go with the A2. Will the standard A2 like RRA sells work well with the mid-length?

RRA

Gas system has no effect on Buffer Spring, Buffer weight/Style, and stock selection. If you are going with a standard A2 butt stock, you will need a Rifle length (also called standard) Buffer tube, buffer spring, and a Rifle buffer.

So basically if you do the A2 stock, avoid anything butt stock related that says Carbine or Collapsible and you'll be okay.

If that is the case (it isn't) then those guys at JP rifles are idiots and/or shysters, and I really don't think so.

EVERYTHING about an AR's gas/operating system affects everything else.

I just put an upper together, 18in mid-length. Runs great on green tip. Bolt won't lock to the rear on ANY milder load, with a rifle or carbine lower. Change to a lighter buffer in a rifle lower and it'll run anything, even some really mild reloads.

So Take a knee, are you saying you are running a carbine buffer and spring in a rifle length buffer tube, or just the the carbine buffer with a rifle spring?

No, I'm saying that "in theory" if you have a complete upper, you can successfully mate it to a complete lower despite the length of barrel, gas system, or butt stock. And should be okay as long as you are running the correct spring, buffer, and buffer tube configuration for that particular butt stock (carbine or standard). Except in the case of carbines where a different weighted buffers may be necessary to get proper cycling in usually "non-standard" calibers or bullet weights that are either extremely heavy or light. That being said, there are always the odd ball case where something else may be effecting the complete operation of the entire weapon (your case for example... maybe) and the components won't work, but you might not know until you try it out either.

But in the OP's question he was asking specifically about the A2 butt stock configuration.

If I understand what you wrote, you have to run a lighter carbine buffer in a rifle lower (with rifle style spring, not carbine spring) to get it to fire all loads? Are you sure you don't have another issue with your upper, like the gas port in the barrel is too small or something in the gas system is not lined up correctly?

"Change to a lighter buffer in a rifle lower and it'll run anything, even some really mild reloads."

I'm lost on this one as well, there is only one A2 stock buffer and spring (specialty buffers excluded) and it's for a rifle gas system. The middy has more pressure, if you have to go to a lighter buffer, something is terrribly wrong.

If you're running a carbine stock on a rifle gas system then, yes it can be tough to find the right combo.

It's a barrel from Montana Barrels so I doubt they screwed the gas port up but that's possible. I double checked the gas block, it the same as a friend has used on several guns so I don't think that is the issue.

Yes, rifle lower, non-full auto carrier with a rifle buffer with the weights removed, runs great.

They screwed the gas port size.

Remember the A2 stock runs a buffer spring 11.750"-13.5" with a buffer that weighs 5.2 oz. The gas port pressure is 13.5K psi in a rifle length system and runs fine as designed with a full auto carrier, millions of M-16A1's and A2's have proven this all over the world.

The middy system taps it's gas at 17K psi.

Think about this for a minute...

Maybe you are right TWR, but like I said, with full house GI ammo it'll run great with any lower configuration. It's only when you drop a grain or so it'll give issue. These are loads that run great in other guns so that isn't an issue.

My whole point is once you change the "recipe", you may have to change something else as well to get it to balance.

Changing the "recipe" has screwed up lots of guns but you have to remember tomato soup has to start with a tomato. The standard is only one weight rifle buffer and spring. If one won't run on the standard buffer/spring, it's time to look for the problem.

JP and some of the others used lighter and heavier buffers, carriers, springs and such to reduce recoil and muzzle jump to increase speed of the follow up shots. A very specialized and finicky proposition.

I do know of one 70's made Colt SP1 carbine that will not run with an A2 stocked lower. Evidently the gas port was sized to run with the lightest carbine buffer but that's strictly on a 16" barrel and carbine gas system.

What's the load it won't run?

The JP stuff isn't finicky, it runs great as designed, their just isn't any "reserve" for a really dirty gun, it'll get sluggish faster than stock. Match guys know how long they can go without cleaning, and if it wasn't reliable, they wouldn't be entering sponsored money matches with something that they thought would choke.

LC brass, WW primer, 55gr Montana Gold FMJ, 25.5 gr TAC.

Have you tried running a different BCG with that upper to see if that could be the issue? I suspect you have, but thought I'd ask anyway.

That load, with the exception of WSR primers, is pretty standard and should run.

"their just isn't any "reserve" for a really dirty gun, it'll get sluggish faster than stock"

That's what I call finicky. There's lots of ways to increase this or decrease that but the one fact I always go back to is how was it designed to work. I don't want to beat my guns to death yet I want them to run and run and run... I've run a low mass carrier, light buffers, heavy carriers, extra power springs, etc., etc. Maybe I just haven't come up with the right combonation that will beat the way it was designed.

Was just reading this and was wondering if this could be part of the problem I saw this weekend. Was with a guy shooting a Bushmaster Predator using some reloads that I had loaded for him. The rifle would not reset the trigger it appeared that the bolt was not cycling far enough to get the hammer to catch on the disconnect. The bolt would pick up another round but the trigger would not be reset. Could cycle the bolt by hand and the trigger would reset. Even had it to double fire a few times...the load that I had loaded was 22gr/H335 68Gr/HPBT CCI400. Could it be that the bolt was not cycling far enough back to make the trigger operate properly?

Yes that was the problem. You were at the starting load for the bullet and powder you were using. You could go to a lighter buffer and makes sure you have the bolt properly lubed or increase the powder charge another grain or grain and a halve.

Possible but sounds like a trigger problem since it doubled, probably a bad disconnector.

Load one round in a magazine and fire it, it should lock the bolt back, repeat 10 times. This will tell if it's a gas/ammo problem or not.

I've not used H335 yet so I don't know about the load but it sounds reasonable since I'm looking at a 28 gr load with 50gr bullets to try myself.

It's funny, playing with different buffers, I've had the bolt beat the magazine follower and skip a round, then I've had too heavy a buffer and wouldn't lock back and was sluggish. Always seems to be the same combo it was designed around that works...

H335 did the same thing in my AR 20 Practical with the rifle length gas tube . I changed to H4895 and it worked fine. One other thing with the Tubb's Spring it wouldn't work either load reliably, went back to the spring that came with the buffer kit and it worked great.

So with the wylde chamber or 5.56nato a person would need to move more to the upper end of the reloaing dat.

Gas systems work their best at certain pressure levels or pressure curves . I would at least go to the medium book level on my loads in the semi-autos or try another powder.

I'd guess trigger issue as well. I actually had this same problem this weekend with an AR I recently built. Well, I actually had a few issues because I was trying some different things. First the AR would not cycle on it's own, too heavy of a buffer spring, but I suspected that this might be an issue before I even shot it. Changed out the buffer spring with a friends 6.8 and it fixed the problem, and his 6.8 works great now too (no twang and almost no recoil now). Then once I got my rifle to cycle, the trigger would not reset. For some stupid reason I never preformed a functions check on it prior to shooting, or if I did (which I cant recall doing) it worked then. Anyway the trigger would reset if I pushed it forward after cycling, so that lead me to believe there was something wrong with the disconnect. On a visual inspection with the upper off you could see it get hung up. I lightly brushed the surface with a fine flat needle file, then some 1000 grit paper, then some polishing compound; no more hang up, and the trigger (RRA 2 stage) is slightly lighter on the first stage now too. Functions check is a go now, so now I just have to get back to a range and put it through it's paces.

H335 is just barely faster than TAC, it should run fine in an AR, as I've used faster and slower... but I tend to load all I can get, no need for light loads.

So I am dumb regards this, I have H2 buffers in one carbine gas and one mid gas, both mil spec buffer tubes, both collapsable stocks, both purchased complete from factory, both work with everything and all the time. I bought an LMT defender 2000 lower with an H3 buffer thinking to use it if I built a 300blackout, I sold it recently because I lost interest in that idea. So it probably will not work with 5.56??? I better call the guy and tell him.

I run H buffers in my middies and have run H2 buffers in my carbines without issue. Set your gun up for 5.56 pressured ammo and the H3 will probably work fine. Throw in some Rem UMC 55gr FMJ and it will probably choke.

It all depends on what ammo the gun is setup for. My first Noveske 18" SPR barrel is an intermediate gas. I figured a standard carbine buffer would be the one to use. I ran some XM193 ammo through it only to have it cycle so fast it would skip stripping a shell out of the magazine toward the bottom of the mag. I called Noveske and was told their barrels are setup to run MK262 ammo in 120 degree heat, ie., lots of pressure. A stuck an H buffer in their as they requested and it ran fine.

Each gun is it's own, a 223WSSM I worked with, needed a 9mm buffer and an extra power spring or it would pull the rims. The guy stuck a Tubbs weight system in his carrier and a Tubbs spring(because someone told him it was what he needed) and had to drop back to a carbine buffer. It was all my fault of course...

So if you was to swap out an A2 stock on a rifle length say 20" barrel gun then you would need to run what type spring and buffer. Man I thought this AR stuff was suppose to be easy..Jezz..

Hard to say, my cousin swapped 2 or 3 of his 24" barreled uppers to carbine stocks and besides his 223WSSM, he had no issues with standard carbine buffers. But never tried one on a 20" barrel.

VLTOR/BCM developed the A5 stock for the Marine Corp. It has been said that a carbine stock on a rifle barrel doesn't work 100% of the time (although Canada's C7/C8 has been doing this for years) and this new stock cures the problems.

I had a 20 inch Fulton armory upper on a RRA lower that shot xm193 fine, I think the original upper was a mid length gas as from the factory. Glad I read all of this for future reference. I may need to tell the fellow I sold the LMT too about the H3.