Mid-length gas systems

C

cmdc

New member
Could someone (or someones), please explain to me what the mid-length systems are all about and why they are better than carbine-length ones? I see references to them in a lot of posts, but I am in the dark about them. I have several 16" carbines; can or do I need to convert them to mid-length systems?
 
Some of the claims I hear are reduced recoil, smoother carrier operation, less muzzle raise, quicker to be back on target. I've only shot one middy gas system and I didn't see any difference from my carbine gas system. I do use a H buffer and it's not over gassed. I'm sure some middy user well add more info.
 
You don't need an AR with midlength over carbine length gas and I sure wouldn't convert one as that would mean replacing the barrel. But if you're buying a new rifle or upper or in the market for a new barrel, definitely look at the middy for the reasons MCB listed above. I have two and am pleased with them.
 
Thanks. I have two stags, and just bought a M&P 15 Sporter, so I'm good for now, I guess, but in the future, I'll go the mid-length route.
 
I am debating over a carbine length vs middy myself. I want a Colt carbine and they only come in the shorty length unfortunately.
 
The differences in gas systems have to do with the length of the gas tube (and therefore, the location of the port). Generally, the longer the gas system, the lower the pressures inside the receiver and the softer the recoil impulse.

The following is generalities, if you want specifics with numbers, it's easy enough to find on the internet.

Carbine length- 7" length, developed for the 14ish inch barrel of the M4, so when you run it on a 16" barrel, you get really high gas pressures in the receiver due to the longer dwell time (the time from when the bullet passes the gas port to it exiting the muzzle). While it works just fine, it also tends to be a little rougher than longer gas systems on 16" barrels.

Midlength- 9" length, basically was developed to match the 16" barrel. The shorter dwell time means there's less gas being shoved down the tube into the receiver and it's at a somewhat lower pressure, giving an easier recoil impulse.

Intermediate- I forget how long this one is, it isn't often seen, but was developed for the 18" barrel length.

Rifle length- 12" length, for the good old 20" barrel.

Another variable is the size of the gas port- a smaller port can cut down on the gas entering the gas tube, minimizing the disadvantages of the shorter systems. Likewise, with a longer gas system on a shorter barrel, a larger port can give enough gas to cycle the rifle (I have a 18" barrel with a rifle length gas system, and it cycles well and is nice and soft shooting).

As said, if you have a carbine length gas system barrel, I wouldn't swap it out (I have one, and I haven't). If you're buying a new rifle or upper anyway, then I'd recommend a midlength over a carbine for a 16" barrel, but it's hardly the end of the world.
 
Thanks, Technosavant. I am thinking that my rifles are probably 16" overall, including the flash hider. That is correct isn't it? So barrel length is probably 14.5". I should probably get a true 16" upper w/ mid-length system sometime just because. I also have a DPMS upper w/ 20" stainless bull barrel that I bought for varminting but haven't used yet. Would it have a 12" system on it. I don't have the guns with me so can't check.
 
^ If you decide on getting a midlength upper, you will no doubt come across Bravo Company (BCM) in your research. Not to tempt you from parting with your wallet, but they just got a batch of midlength uppers in and they sell out within days, sometimes hours. As of the time of this post, they have both 14.5" midlengths and 16" midlengths in stock, with choices of handguards/rail systems or without. Check them out when you have a chance.

And no, I don't work for 'em. ;)
 
I am thinking that my rifles are probably 16" overall, including the flash hider. That is correct isn't it? So barrel length is probably 14.5".
Click to expand...

Maybe, maybe not. If the flash hider is permanently attached, then it's likely a 14.5" barrel. If not, then you're pretty well guaranteed it's 16". Generally, 16" models are way more prevalent due to the buyer basically having to marry the flash hider on the shorter barrels- getting it swapped out can be done, but requires a competent gunsmith. Most folks don't quite want to make that commitment, so the 16" models are more popular.

The two are visually distinctive, if you look at pics of a 14.5" barrel and a 16" barrel, that extra inch and a half is easily noticed.

The recommendation for BCM is good- their stuff isn't as cheap as the major brands that sell in volume, but their quality is unsurpassed; I have an upper that started life as a 16" carbine length gas system BCM... it's since been chopped back nearly a couple inches and had the suppressor mount permanently attached.
 
Thanks, guys. I'll look at them when I get back home. On vaca. now.

Now, for the BCMs: I just looked at their website, and am confused as to which one I want. They have several 16"s listed, but I don't know one from the other. 10", 12", does that refer to the gas system. and if so, which one is better? They are both referred to as mid-length. Lightweight, etc.? I have read a number of these posts, and BCM gets great reviews from everyone.
 
That measurement probably refers to the length of the handguards.

The product descriptions on BCM's website for their uppers go like this:

[Barrel length][Gas system length][Handguard/Rail system]

For example: "BCM BFH 16" Mid Length Upper Receiver Group w/ Vltor VIS 10"

That's a 16" barrel with midlength gas system and a Vltor VIS 10" rail system.
 
cmdc, I recently bought a Daniel Defense 16" LW CHF midlength upper and am pleased with it. It is 16" plus the flash hider which adds about 1.5" to the barrel. The gas system is the 9" midlength of course and the LW means a thinner barrel that weighs a few ounces less than the standard govt profile which is thin under the handguards then thicker forward of the gas port. CHF is cold hammer forged which is a plus but not necessary.

Like CortJestir and Technosavant said BCM is an excellent choice as well. Just be sure you choose the right barrel length, I would recommend 16" so you don't need a permanently pinned muzzle device but many love the 14.5" With midlength gas you'll probably only see 16" and 14.5" barrels, anything shorter probably would have a carbine or pistol length gas system.

When you buy an upper from BCM and many other vendors remember you'll need to think about adding a BCG, charging handle, handguards/rails and maybe a rear sight. Lately BCM has run out of stock quickly on uppers and BCGs so don't dally if you see something you want. Or get on their email notification for OOS items.
 
Thanks, guys. I'm lookin' at the BCM sight in earnest. Thanks for the heads-up about the BCGs for those uppers, I thought they were a complete setup.

As Floyd the barber says, "it's funny the things you don't know."
 
Thanks for the heads-up about the BCGs for those uppers, I thought they were a complete setup.
Click to expand...

Usually, the terminology used can hint at what it is.

"Complete upper" usually means an upper that is 100% ready to go, with all parts (handguard, gas tube, BCG, etc.). May or may not have the rear sight, but otherwise it's pin it to a lower and go.

"Barreled upper" usually means you have the upper receiver with small parts installed, barrel affixed, and may or may not have the gas tube assembly. Probably will not have the handguard, BCG, or rear sight.

"Stripped upper" is usually the bare upper receiver, no barrel, small parts, etc. There are also bare uppers with the small parts (forward assist and ejection port cover) installed, but no barrel.

BCM sells their uppers as barreled uppers- no handguard, charging handle, or BCG. They do sell such things, but things like their BCGs are in great demand and may not always be in stock.
 
The longer gas systems are also gentler on the action. Since the gas pressure is lower with midlength, it puts less stress on the bolt, extractor, etc. compared to carbine length guns. Which means higher reliability.
 
An important aspect to keep in mind when evaluating midlength vs. carbine gas systems is that we are talking very small differences here. If you aren't shooting tens of thousands of rounds of 5.56mm, you probably aren't going to notice a difference in wear and tear.

The main things I like about the midlength are having more handguard real estate (less barrel branding, can use a more forward grip) and having a better sight radius for irons (although since I use optics 90+% of the time, kind of a moot point).

If I had a carbine already, I would probably keep shooting it until I wore it out and then switch to midlength when I swapped barrels (which is exactly what I did with my last carbine length gas system).
 
Thanks, guys. I appreciate the information and advice. I can't tell you how much money I've spent upgrading to the latest and greatest only to find out later that it wasn't really necessary. As they say, if it ain't broke, don't fix it. That's why I have learned the hard way to ask here before I jump off the deep end of the pool. I'll keep what I've got until I wear it out, which at the rate I'm going is going to be in the next lifetime, anyway.
 
mid length

My favorite configuration is a 16" heavy barrel with a mid length gas tube on a fixed A-1 stock. Fits me perfectly. I have carbines and a 20" rifle but they mostly stay home. The mid length stays in my truck.
 
I'm really tempted to install an A/2 stock on my RR mid-length......simply and solely because of the handling characteristics. Far as the mid-length itself goes I believe it's more an aesthetic thing than any real advantage. I do know that mine has a lot more solid and balanced feel (to me) than carbine setups I've owned in the past, and that ML version does give you a lot more territory hold on to. Visually tho, at least to me, the ML does present a lot more balanced package.

FWIW, I built mine from scratch using all RR parts, less the carrier, which is an extra 16 version I just happened to have on hand. The BBL is RR's stainless HB 1/8 version with the Wilde chamber and will hold sub minute groups all day long. That collapsible stock is a plus for carting it around, but as I said, from a using perspective I truly think I'd be better off with the standard version...then again, beauty is in the eye of the beholder isn't it!

Probably worth adding that that 1/8 twist really surprised me by shooting as well as it does with everything from 40 to 70 gr bullets..........it eats whatever it's fed.

Downside of the Wilde chambering is that if one happens to have a sub caliber device of Atchison or Ciener manufacture (may god help you in the latter instance) the shorter chamber MAY create an issue with the chamber insert.......mine works most of the time, but it's no where near as well fitted as in the Colt......chamber insert protrudes sufficiently to affect feeding reliability.
 
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