carbine length vs mid-length gas system on 16 in barrel? Is the amount of over gas so much on the carbine length that the gun will beat itself to death or is it an acceptable amount of gas? What are signs of over gassing?
 Thanks

Both work,but I prefer mid.

thanks

A properly built 16'' carbine length will run just fine and not be over gassed. A poorly built 16'' middy can be problematic and over gassed. It has a lot to do with the gas port diameter. You can usually tell if a rifle is over gassed by the ejection pattern and the recoil impulse. You can also go a step further and buy a bunch of buffers, carbine, h1, h2, h3 and do some experimenting. Try not to think so hard on the gas system as opposed to buying quality parts from people who know what they are doing. That said, I prefer 14.5 middy from bcm.

There is more to gas system function than the quantity of gas.

In the AR15 design, the timing of the gas impulse is also a factor.  Changing the gas port cannot change the timing - only the gas system length to barrel length proportion can do that.

Midlength is a better option for a 16" barrel.

fair enough

ALL AR15 DI systems are over-gassed.  ALL of them.  The design requires it.  There is more gas impulse than necessary to cycle the action, which allows the action to cycle reliably under adverse conditions or with different ammo types.

Whether it is carbine, rifle, or midlength gas system length, does not really have much bearing on whether the AR will be more or less over-gassed.  As said above, the length of the gas system has more to do with the timing of unlocking the bolt, and the length of barrel in front of the gas port, along with gas port size, have to do with the length of the gas impulse (dwell) and the amount of gas allowed into the DI system (port size).

This is all a relationship, and part of that relationship is the reciprocating mass (mass of the carrier and buffer) along with the action spring weight.


That's why it is important to buy from reputable manufacturers who understand, test, and design for these relationships when buying factory rifles.  When buying parts, it is goof to purchase from reputable manufacturers who will properly size the ports, ship quality buffers and springs, etc.

Even some well loved manufacturers WAY over-gas their components (or they get them this way from their suppliers) such a BCM (large gas ports).  I cant speak for why they do this but common perception is because they value reliability over less recoil impulse.... or they do this because so many shooters today use inexpensive underpowered non-milspec ammo (Wolf/Tula) which requires significant over-gassing to reliably run these out of spec ammunition types.


The biggest benefit of a midlength over carbine length gas system in a 16" barrel, is that there is a longer time for the initial gas impulse will take action on the carrier, unlocking the bolt and starting extraction, and a shorter dwell time due to less barrel length in front of the gas port.  This results in softer/easier extraction, and a softer overall recoil impulse, assuming gas port sizing remain constant for the standard size in that particular configuration.


Both systems can be tweaked, by adding heavier springs and buffers, or adjustable gas blocks.  Or just left alone if they work and the recoil impulse is not bothersome.

My S&W works great and recoil is not bad I guess if it aint broke dont fix it

Over gassed rifle may exhibit one of these symptoms...







Head swipe looks like a crescent shape gouged into the brass



and/or



Rounds ejecting forward of your firing position.



and/or



Case separation about 1/2" above case base.



and/or



primers popped out when fired.



Plus a variety of other indicators

idk about ejection pattern but I'm checking brass now and it looks clean

If you go with a quality maker, that has enough insight to optimize the gas system of their rifles, either one would be better than either of a assembling company.

Many rifles are made overgassed to avoid the cheap underpowered (like steel cased) ammunition, or from failing when improperly maintained. Rifles for dummies.

An LMT MRP 16" barrel has a midlength gas system. That speaks to me, since it is their own innovative design and has proven itself pretty well.

The 14.5" MRP barrel is a carbine length gas system.

People here LOVE this chart.

EDIT: Actually useful. I had an SPR type build that had a really hard initial pulse, especially for how heavy the rifle is. Also, it was throwing brass as far forward as I'd think possible. Since I didn't want the gun to beat itself, or me, to death; I replaced the YHM gasblock with an adjustable one from Seekins. Again, I'd bet YHM opened their gas block way up so that it wouldn't cause failures and get returned. I wouldn't be surprised if Wilson Combat had done the same thing with the gas port on their barrel.

Thanks!!!!

lol on the left side.

I think we can sum up all of your concerns...

1.  The M&P 15 Sport is a good rifle for the money spent.  Buy ammo, mags, and range time.  With the proper ammo, it will serve you well for target shooting, hunting, HD, and SHTF.  No need to wonder if you purchased a decent quality rifle.  If something is wrong with it, S&W will make it right.

2.  If you are going to shoot NATO spec 5.56 only, you could possibly benefit from a heavier buffer.  If you are going to shoot Tula, or .223, use the heaviest buffer that will run reliably with the weakest ammo you plan to use.

Thank you I am looking into getting an H buffer

I've got my mid-length on an A2 upper. I really like the longer site radius and if I should ever have the need, I can put a bayonet on it.

No kidding damn

I'd recommend getting an H3. You can swap out a tungsten weight for a steel weight to make it a H2 if it's to much buffer. (see pic below, obviously, a H3 is one with 3 tungsten weights)

It's not to get a better recoil impulse or stop the weapon from beating itself up, it's to make it function/cycle better. "better" means- a heavier buffer will delay bolt unlocking allowing chamber pressure to drop to a level that will allow more reliable extraction. A carbine gas system is more prone to these issues because the gas impulse reaches the carrier faster and if the port is on the larger side (over gassed) it exacerbates the issue.

FWIW I've never had AR reliability issues until I started messing with buffer weights. Ymmv

This.  If it reliably cycles all your ammo, it should probably be left alone unless it is being converted into a race gun, 3G competition or such.  Far better to be a little overgassed than to encounter reliability problems with ammo variations.