AR15: Should I go Midlength?

[Varminterror]

^ JP adjustable gas block/front sight is GTG.

 

[Riomouse911]

I seem to recall talk of a new GuyCard guideline that states that those of us who have ARs must have at least one of each gas-system length.

Sorry ... feeling a bit silly at the moment ...

WHEW! Got that requirement covered!

 

[cougar1717]

There is nothing wrong with the 6920 to make anyone doubt anything. Just run it until it starts keyholing. By then, you will have spent more on ammo than what was spent on the 6920. It will tackle the carbine class just fine.

But that is not really the question, is it? It is more about wanting train with a new "top of the line" BCM rifle for the carbine class, right? And then, recommendations for that purchase. That, dear sir, depends on your preferences and pocketbook.

It is difficult to make suggestions without knowing the user's preference as there are many AR configurations, but I would not switch out of the 6920 solely due to carbine vs mid length gas systems. I would use the carbine class to see what others are using and gain preferences for grips, triggers, handguards, sights, etc. Maybe someone will let you hold or try their rifle at some point. From that experience and knowledge base, I would make your BCM purchase.

 

[MistWolf]

I have a Colt LE6920...Are the advantages of the mid length enough to get a new midlength carbine/upper?

No

Or would I be better off spending the money on ammo and practicing with the Colt?

Yes.

That's the short answer. The long answer is, shoot your Colt until you have answered the questions for yourself.

There is something you can do to make a small improvement to your Colt. Get an H2 buffer and a Sprinco blue spring. Neither cost much and your Colt will run a little bit smoother with 5.56 spec ammo without affecting reliability. However, it's not necessary to get the H2 buffer or the blue spring.

More importantly, before going to class, get a Vickers Blue Force sling. I've tried a few different slings and the one that worked best under the tutelage of a shooting instructor was the Vickers. If you can't get a Vickers sling, almost any sling will be better than no sling. You'll be hating life if you go without a sling.

You can successfully attend a class with iron sights, but an RDS makes it easier to apply the lessons. Most classes assume the student will have an RDS on their AR. Aimpoint dominates the market for good reasons. However, if you cannot afford an Aimpoint, get a Primary Arms Enhanced Micro. There are two versions. One uses a dial to adjust illumination. The other uses push buttons. The dial version is MUCH easier to use.

Take a look at the MagPul Slimline handguard. They are inexpensive, tough, comfortable, insulate against heat and are longer than other handguards. The extra length lets you get your hand out further and covers the gas block to reduce the chance of getting burned. Again, not necessary, but I can tell you from experience, running an AR hard is much more pleasant with them.

But when all is said and done, never forget- Ammo Before Accessories. Shoot Before Modifying

 

[MistWolf]

Yes - throttling the gas block reduces volume and pressure, but opening it up gives you the option to take that back for lighter/faster bullets or lighter loads. It’s a longer gas impulse since the dwell time is longer, but the AGB gives you pressure over volume and pressure.

It’s not EXACTLY the same combination of impulse duration, pressure, and volume, but the behavior for action cycling is really the same. And in buying a midlength barrel, you only have that ONE set of system parameters, which may or may not match your actual load. An adjustable gas block is more versatile, as it remains adjustable. Pretty hard to engineer an adjustable length gas system.

Gas ports and adjustable gas blocks are a restriction. Neither regulate pressure. Restrictions control how much volume will flow within a certain amount of time at a certain pressure. Fluid will flow through a restriction until the pressure is equalized on both sides. At that point, flow will cease. For example, if there is 20k psi on one side of the restriction and 0 on the other, fluids will flow through the restriction until the pressure is the same on both sides. If the system is sealed and the volume of the vessels is the same on both sides, flow will cease when the pressure is 10k psi on both sides. What the restriction determines is how long it will take the pressure to equalize. A smaller restriction means it will take longer than if a larger restriction were used.

What controls the operating pressure in the carrier is the carrier and action spring. When the expansion chamber reaches operating pressure, it opens, venting excess gas to the atmosphere. No matter how pressure reaches the expansion chamber, it will open when operating pressure is reached. The rest gets vented. The job of the gas port/adjustable gas block is to control the flow so the pressure is reached within a certain time- not too soon. Not too late.

 

[Varminterror]

@MistWolf - I’ve been through the transient state fluid mechanics on this site before, maybe even with you. Do some studying and get back to me. The restrictive oriface does pressure. That’s first year engineering school stuff.

What you say is true in a continuous flow CLOSED system, which a firing rifle is not. The pressure is NOT equalized in the transient case, and the restriction abates pressure.

 

[Shawn Dodson]

The biggest issue I had with my 16" carbines was muzzle jump. I experimented with different buffer weights, recoil spring combinations and muzzle brakes. I settled on an H buffer, a red Sprinco spring and a VG6 556 Gamma muzzle brake. My carbines now have zero muzzle jump and negligent recoil.

(FWIW, I also tried the JP Enterprises A2 adjustable gas block and I did not like it. Even with the prescribed #243 Loctite the setscrews would work loose. A great big NO-GO for me. In addition to the cost of the A2 gas block you have to buy a bottoming tap to thread the gas tube to accept the adjustment setscrew and the Loctite.)

I have one 16" 5.56 midlength carbine and an 11.5" 5.56 midlength pistol, and two midlength uppers (25-45 Sharps and 6.8 SPC). I don't perceive any difference between them and my 16" carbine length carbines, except the 6.8 has a little more recoil. If you're happy with your carbine then if I were you I wouldn't waste the money to chase the alleged benefits of a midlength gas barrel.

 

[MistWolf]

@MistWolf - I’ve been through the transient state fluid mechanics on this site before, maybe even with you. Do some studying and get back to me. The restrictive oriface does pressure. That’s first year engineering school stuff.

What you say is true in a continuous flow CLOSED system, which a firing rifle is not. The pressure is NOT equalized in the transient case, and the restriction abates pressure.

Whether the system is open or closed, the laws remain the same. Pressure flows from the high side to the low side until pressures equalize. The gas port diameter along with pressure controls flow. It can delay how long it takes the pressure to equalize, but it does not regulate or change pressure. I deal with this everyday in aircraft systems.

 

[md7]

OP,

I own Carbine and Mid length variants. Personally, I can not tell a difference in recoil. Both shoot lower pressured .223 and hotter 5.56 just fine and have done so with both types in below freezing and very hot temps.

If you shoot irons, the mid length will help some due to having a longer sight radius. For me, that’s about the only real benefit of the two that I can discern.

I’ve gotten to the point where I care more about material and assembly than gas system length. Either and both are fine with me, and your 6920 meets those criteria in my opinion.

 

[MachIVshooter]

Whether the system is open or closed, the laws remain the same. Pressure flows from the high side to the low side until pressures equalize. The gas port diameter along with pressure controls flow. It can delay how long it takes the pressure to equalize, but it does not regulate or change pressure. I deal with this everyday in aircraft systems.

Therein lies the time component that cannot be ignored; port pressure is the same, regardless of orifice size, but the pressure which builds in the gas system is limited by the dwell time.

 

[Hokie_PhD]

It’s possible that I’ll be getting into some 3 gun competitions. Overall though this is a “go to” rifle. I would like to be able to practice a lot.

If you haven’t done 3 gun the standard recommendation is to use what you have first. Figure out what you want and need after that and get that gear after a few matches.

An alternative is use this as an excuse to build another AR. In this case build another upper then a lower. Go as long as you can, an adjustable gas block, get the twist rate that works best for you and get a 16” or 18” barrel (or one of each).

I went the later and documented my budget 3 gun build. I’m still in need of an optic and time at the range to get it broken in and practiced with it. In my case I knew my PSA carbine wasn’t what I wanted so I used this as an excuse to learn to build an AR.

 

[Varminterror]

@MistWolf - an intelligent conversation can’t be sustained at this point. Pressure doesn’t flow anywhere. Mass flows under the force of pressure. Learn a bit about gas let down systems and you’ll realize the operating paradigm for the AR action and gas port oriface restriction. Hell, google fitting loss and pressure drop and you’d learn enough in 5 minutes to re-evaluate your overly simplified view of the dynamic case we’re discussing.

 

[Jackal]

If you blindfolded most of the MidLength bros, they'd never know the difference between the carbine and middy. I've assembled and shot dozens of AR's and cant tell a difference between gas lengths. The major difference seems to come from the usually higher weight of rifles with longer gas systems.

 

[MistWolf]

Therein lies the time component that cannot be ignored; port pressure is the same, regardless of orifice size, but the pressure which builds in the gas system is limited by the dwell time.

That is correct.

 

[MistWolf]

@MistWolf - an intelligent conversation can’t be sustained at this point. Pressure doesn’t flow anywhere. Mass flows under the force of pressure. Learn a bit about gas let down systems and you’ll realize the operating paradigm for the AR action and gas port oriface restriction. Hell, google fitting loss and pressure drop and you’d learn enough in 5 minutes to re-evaluate your overly simplified view of the dynamic case we’re discussing.

...and what happens when that mass flows? Pressure drops on the high side and if it's a closed system, rises on low side until the pressure is even. If it's an open system, pressure on the high side drops until it matches atmospheric pressure.

I don't need to "Google" anything on the subject. I've got training and tech data from the Air Force, McDonnell-Douglas, Boeing, Lockheed and Sikorsky (just to name a few) to work from.

 

[Varminterror]

As @MachIVshooter mentions - the time aspect cannot be ignored. @MistWolf is making the common mistake of assuming steady state, closed system rules apply to a transient state differential pressure system.

Gas letdown systems inundate the production world. You’re wholly neglecting fitting loss in a transient system.

 

[someguy2800]

The pressure will never equalize because the dwell is too short to fill the carrier and the gas system is not sealed so its leaking out from both ends of the gas tube and bolt. The pressure reached in the carrier will be a fraction of port pressure so restricting the gas port will change the pressure reached in the carrier. Your right that it does not “regulate” pressure, but it absolutely will change the pressure in the carrier.

 

[Everready73]

There is nothing the Colt cant do that a mid-length can. Buy more ammo and practice, it will be money much better spent. Take a training class instead as well.

Technically a midlength is better and if i had 1k or more to spend i might go with a midlength bcm or the Colt CCU, i would have no problem with a 6920 being my only rifle

 

[Charlie98]

I've got 2 middy AR's, a rifle, and a carbine... TBH, I can't tell the difference. What I do like is the extra length of the forearm the mid-length system gives me over the carbine. As far as all that technical stuff you guys are gassing on about... at the operator level all I care about is that they work, and I think AR tech is good enough at this point that even a pistol-length gas system is reliable.

Buy more ammo and practice, it will be money much better spent.

+1

 

[bearcreek]

I like mid length gas systems but it's not worth selling a good rifle just for that unless, as others have said, you just want a new rifle. One thing that has been mentioned as a benefit of mid length several times that is easy to retrofit onto a carbine is a longer sight radius. You can install a low profile gas block and free float rail and have whatever length sight radius you feel like.

 

[SamT1]

I don’t believe I own a mid length, I’d have to look. But rifle, Carbine and pistol length. The rifle is easily the smoothest. I don’t shoot competitively so I don’t know if that translates to quicker hits for sure, but I believe it would.
They are all dependable with everything from the cheapest steel case I find to my high end hand loads loaded at mag length.
There’s enough difference in rifle and carbine to be a good excuse for buying another! Though I like the short and light Carbine rifles.

 

[Coal Dragger]

I have a Colt LE6920 I purchased about a year ago, of course with the carbine length gas system. I thought I had the last AR I’d ever need, but reading about these midlength gas systems has started to make me doubt. If I really want a top of the line AR, should I have gone with midlength? Now I’m considering getting a BCM middie upper, but then I’d be tempted to just get a whole new middie rifle and leave the Colt as is. And THEN I started lookin at Mk12 uppers with 18” barrels and rifle length gas systems, which should be even more reliable and durable than the midlength. However, perhaps an 18” barrel isn’t optimal for home defense/general use, which is what my main “go to” AR is slated for. I do like the idea of a Mk12 upper though. There’s actual data by the military showing the advantages of rifle length gas over carbine, unlike middie.

Are the advantages of the mid length enough to get a new midlength carbine/upper? Or would I be better off spending the money on ammo and practicing with the Colt? It’s never had any problems, but I’m going to a 1,500 round carbine class soon.

Take your 6920 to class, it will do quite well.

A few years ago I attended a Pat McNamara class and watched one of the shooters literally take a brand new 6920 out of the box, throw on an optic, sling, add lube generously, and run great in the 2 day class. Day 1 first portion was zero and basic rifle marksmanship so this guy broke his 6920 and optic in at the class.

Yes a mid length will shoot a smidge softer if the gas port is properly sized, and yes a carbine length gas can be way over gassed. Fortunately Colt properly ports their 16” carbine length gas barrels, and include an H or H2 buffer, el-cheapo brands don’t buy you got a quality carbine.

So my advice is take that money you were going to throw at a new upper and get more ammo. Maybe a a Geissele SSA, or SD-C trigger too.

Otherwise shoot the gun.

As for .mil users and the mid length take note that USASOC (US Army Special Operations Command) is running an upgrade program on tethering M4’s. A Geissele MK16 rail, and Daniel Defense 14.5” cold hammer forged mid length barrel are the major component changes, along with a Geissele Airborne charging handle. The hammer forged mid length barrel was found to put up with the idiocy that is M855A1 ammo far better than the previous button broached barrel.

https://geissele.com/urg-i-complete.html

You can buy one for a lot money, or just enjoy your Colt until the unlikely event you shoot the barrel out.

 

[GBExpat]

Clever!

*I* think that most folks only need a $450-$500 AR, maybe a ½ case of plinking ammo (say, $150) and, if they are not familiar with the AR, a good friend who is and who is willing to spend some time teaching them the ropes, so to speak.

After that if they decide to seek some professional training, fine ... if not, also fine.

 

[Coal Dragger]

A sub $500 AR is usually garbage.

 

[GunnyUSMC]

A sub $500 AR is usually garbage.

My buddy put his 19 yo grandson in an AR class and had him use his 6920. The Colt started chocking early the first day.
Luckily I had just built my buddy a PAS PTAC carbine for $450 to use as a truck gun and he had it with him. His grandson finished the class with the PSA, putting over 1000 rounds through it with no problems. Those cheap AR’s sure are garbage