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AR15: Should I go Midlength?

Discussion in 'Rifle Country' started by BluedRevolver, Jul 26, 2018.

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  1. BluedRevolver

    BluedRevolver Member

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    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.
     
  2. someguy2800

    someguy2800 Member

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    I think your overthinking it myself. I'm not an AR expert but I have owned and shot a couple of both and the only difference I could tell is the mid-length cycles a tiny bit smoother. I wouldn't change a rifle I already had but if you want to get a different upper for some reason, like for example if you want an upper with a red dot or optic on it, I would always get a mid-length upper if given the choice between the two.
     
  3. chicharrones

    chicharrones needs more ammo

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    I don't have the answer you are seeking, but if you like the Mk12 upper, I'd get or assemble a whole new gun to get yourself a good facsimile of the Mk12.

    That way you'd still have your Colt carbine and a new rifle.
     
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  4. MachIVshooter

    MachIVshooter Member

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    Not unless you're just looking for an excuse to buy another AR. I definitely prefer midlength gas for 16" guns, but I wouldn't spend money changing an otherwise good running carbine just to get the slightly gentler operation.
     
  5. Justin

    Justin Moderator Emeritus

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    Unless you're planning to shoot competitively, there's not much of a point to going with the longer gas system.
     
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  6. BluedRevolver

    BluedRevolver Member

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    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.
     
  7. GBExpat

    GBExpat Member

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    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. :D

    Sorry ... feeling a bit silly at the moment ... :)
     
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  8. Varminterror

    Varminterror Member

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    I’m not a fan of “overlength” gas systems unless I’m competing and the rules preclude AGB’s. An adjustable gas block can make a short system act long, but the reverse is not true. If a mid-length doesn’t run how you want for your load, there’s not much you can do. With shorter gas + AGB, life is simple.
     
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  9. Bwana John

    Bwana John Member

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    For shooting iron sights my older eyes notice quite a bit of difference between carbine and midlenght sight picture.
     
  10. JeeperCreeper

    JeeperCreeper Member

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    http://soldiersystems.net/2018/05/1...s-system-testing-shows-increased-performance/

    Yes, midlength "should" be better, but I'd rather err on the side of quality. That Colt should last you a long long time with little to no malfunctions. If you spend enough money on ammo to burn out the barrel, just but a new barrel. If you haven't had malfunctions, I wouldn't fret.

    But if you really have want a middie, then get yourself a middie. I have a PSA midlength that is my beater. Runs great, just like the carbine.
     
  11. BluedRevolver

    BluedRevolver Member

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    This is the first time I’ve heard a statement like this. One can obtain the benefits of a longer gas system on a carbine length using an AGB? Would I be able to change the A2 FSB of my LE6920 to an AGB without much issue?
     
  12. MachIVshooter

    MachIVshooter Member

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    Sort of. You still have higher port pressure with shorter gas systems. By restricting the aperture through which it flows, you can reduce the total volume of gas entering the tube and cylinder, and some of the pressure will attenuate as it fills the available volume, but it's still not quite the same as a gas system length most appropriate to the barrel length. Think of it like electricity; just because your total wattage is where you want it doesn't mean the balance of volts & amps is ideal.
     
  13. bfoosh006

    bfoosh006 Member

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    IMHO, a better example of how a AGB works is... A garden hose v. a firehouse... both have the same pressure, just different volumes of said pressure.
    Remember, Colt Carbines have successfully run for decades... and a Middie is a better mouse trap... but not enough to make me want to change needlessly.

    That said, every new upper I have bought has been a minimum of a mid-length gas system.
    Middies work at lower pressure points on the barrel.

    Even the carbine length GS ones ( 5.56 ) I have tried an AGB on really didn't benefit "dramatically" recoil wise..
    In a .308 / 6.5CM AR ... I always install AGB's ...HUGE difference in those.

    IMHO, a stronger recoil spring and a heavier buffer weight is a practical approach to a carbine GS to help "slow" down the cyclic speed.

    IF the OP really wants to "feel" the difference between a carbine and a middie GS... I'd ask some one at the range next time.

    If the OP wants... PSA sell's all sorts of complete uppers in Mid Length gas... at VERY reasonable prices.
     
  14. edwardware

    edwardware Member

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    Don't forget a middy mock dissipator too! No collection's complete without one.

    (Then you'll be needing a real dissipator with a rifle length system, just to compare of course.)
     
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  15. BluedRevolver

    BluedRevolver Member

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    I’ve shot a middie before and didn’t notice a difference, but that could be because the front end was quite light with a pencil barrel. Only time I noticed a difference from my carbine was shooting a friend’s AR with an external piston. That did seem to shoot softer. I have no real desire for a piston though. I don’t even really have a desire for a mid length, but I do want to have a proper go to rifle and if the mid length is demonstrably more reliable and durable than it’s something I’ll need to consider.

    However, I’ve heard mixed things. While most say middie is undeniably better, others say they are more picky on ammo and that the carbine will function more reliably when dirty and fouled because of more gas pressure. Any truth to this? Are mid lengths more picky on ammo and/or being clean?
     
  16. MachIVshooter

    MachIVshooter Member

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    No, mid length is not more temperamental. Carbine gas on a 16" barrel is more apt to run with significantly underpowered ammunition, but it's also more violent with ammunition in the normal pressure range, since the higher port pressure and excessive dwell time renders them "overgassed".

    All else equal, you can expect longer bolt life out of a middy system than a carbine system, but most shooters will never wear out a bolt in either, so that point is largely academic. There are hundreds of thousands of 16" guns with carbine gas out there running just fine. Like I said in my first reply, unless you're just wanting another upper/rifle, your money is better spent elsewhere.
     
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  17. BluedRevolver

    BluedRevolver Member

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    Another question... if I went for a midlength, should I just spend the $550 for a BCM complete upper and slap it on my Colt lower, or is there some advantage to buying a complete BCM mid rifle (other than having 2 rifles instead of one, haha). I’d be tempted to save the money from just buying a complete upper and spend the rest on ammo. A complete mid 16 BCM rifle is nearly $1200 from a glance around.
     
  18. taliv

    taliv Moderator Staff Member

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    Having built and shot dozens of them in competitions and classes in the past few decades I would definitely prefer an 18” bbl and rifle gas with a 1-6x or 1-8x optic as my do it all.

    It’s substantially better.

    That said a colt 6920 in factory config with a red dot is light and fast and just easy. It’s not gonna have the same split times or long range exterior or terminal ballistics but it is good enough for gov work and nothing beats the reliability.

    I’d never tell anyone to spend money on guns instead of ammo. But if you can do both then get an 18” build. I’d put a nicer trigger in it than the factory colt too. Bring both to your class and swap back and forth and see which one you like better.
     
  19. BluedRevolver

    BluedRevolver Member

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    Can I put a Mk12 upper on a standard 6920 carbine lower? I see that both Daniel Defense and BCM have their complete Mk12 rifles with carbine collapsible stocks. Looks like a carbine buffer system. Is this good for a rifle gas system and 18” barrel?

    Getting a Mk12 upper does seem like more of an upgrade than a middy upper. From the answers here I’m getting it appears that it’s a wash between carbine and mid, with Colts being reliable and durable regardless. I do like the way the carbine looks better. I shoot well with it, and I suppose I could always get a midlength upper to stow away to replace the Colt carbine upper when/if it wears out. Would I need to worry about the Colt lower wearing out faster with a carbine gas system? Then again, there’s XM177E2 lowers from the early 70s thatre still kicking on full auto with everything replaced but the lower. As I understand it, lowers don’t really wear out right?
     
    Last edited: Jul 26, 2018
  20. taliv

    taliv Moderator Staff Member

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    Yes you can. The only issue is that you still have an extremely reliable trigger that sucks for long range precision shooting. Since the 18” bbl is much more capable of hitting smaller targets at distance you’d want a trigger to match.

    If you’re going to stick with short range targets. Don’t waste the $ on the longer bbl.

    You can still make hits with a crappy heavy trigger. You just have to practice a lot and have good trigger control
     
  21. GunnyUSMC

    GunnyUSMC Member

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    The Mid length gas system was developed to over come the over gas problems with the carbine gas system, but the military stayed with the carbine system.
    The two main advantages to the mid length gas system are only gained if you are using it with a front sight tower. You get a two inch longer sight radios and you can mount a bayonet on the rifle. If you go with a rail, you just get the slightly smoother recoil.
    I have both. The rifle length system is a joy to shoot.
     
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  22. someguy2800

    someguy2800 Member

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    That pretty much answers the question.

    I think what you have is just fine and is not worth the money trying to fix what is not broken. If all your going to do is shoot short range with iron sights just leave it alone and enjoy it. If you want to shoot a little longer ranges with an optic then consider getting a separate upper with either a 16" mid-length or 18" rifle length barrel and free float handguard. The BCM or whoever elses upper will click right on your colt lower. Takes 30 seconds to switch. Then if you decide you want to get a different lower later with a better trigger or butt stock you can add that to the roster and have two separate rifles.
     
  23. SamT1

    SamT1 Member

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  24. Varminterror

    Varminterror Member

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    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.
     
  25. someguy2800

    someguy2800 Member

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