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  1. #1
    Private jcsti's Avatar
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    Default 223 lower w/ 556 upper

    I'm looking to do a build and I'm wondering if there will be any problems matching a lower 223 with a 556 upper reciever? thanks

  2. #2
    Captain Brad@AST's Avatar
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    No problem at all.
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  3. #3
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    The lower has nearly nothing to do with the caliber for the upper. For my 5.56 lowers, I have a .300FB and 6.5 Grendel. Any caliber that has a magazine that fits in the magwell could theoretically be used with any AR-15 lower, aside from some of the pistol caliber only magwells.

  4. #4
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    Most lowers are marked Cal: multi.

  5. #5
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    I never knew the lowers were marked for calibers, not the 5.56/.223 ones anyway. I do know that the 308 requires a special lower sure to the magwell difference.
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  6. #6
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    The only thing that matters with 5.556/.223 lowers is whether it has the m4 feedramps, it is said you can use a m16 style lower (without feedramp cuts) with an m4 feed ramp upper but the other way around can cause problems.
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  7. #7
    Sergeant zeeto's Avatar
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    Quote Originally Posted by Sprout47 View Post
    The only thing that matters with 5.556/.223 lowers is whether it has the m4 feedramps, it is said you can use a m16 style lower (without feedramp cuts) with an m4 feed ramp upper but the other way around can cause problems.
    You're confusing the upper/barrel with the upper/lower on this.

    If the upper has the m4 feedramps then the barrel should; vise-versa it doesn't matter.

  8. #8
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    i have not seen a lower marked .223, but many marked 5.56
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  9. #9
    Colonel Sprout47's Avatar
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    Quote Originally Posted by zeeto View Post
    You're confusing the upper/barrel with the upper/lower on this.

    If the upper has the m4 feedramps then the barrel should; vise-versa it doesn't matter.
    You're right lol - long long day

    and just above, I think Bushmasters say .223? My Colt sporter says .223 but the barrel was 5.56 NATO.
    Last edited by Sprout47; 01-22-2011 at 09:23 PM.
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  10. #10
    Administrator Jeff's Avatar
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    Sabre Defense lowers says "5.56MM NATO" on them.
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  11. #11
    Private jcsti's Avatar
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    Quote Originally Posted by KeithB View Post
    i have not seen a lower marked .223, but many marked 5.56
    its a preban colt carbine lower marked .223

  12. #12
    Major tinker's Avatar
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    I think most of mine are marked ".223/5.56", I know at least one is marked "multi".
    But it really doesn't matter what the lower says, the chamber of the barrel will be what dictates what ammo the rifle will shoot.

  13. #13
    Colonel Tifosi's Avatar
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    Ah hell, at what point is some backroom anti at whatever office building going to come up with a stipulation that you can't have 5.56mm because it's military, but .223 is OK because its sporting/varmint?

    At which point would they force inspection of any rifle that a LEO came across using specific chamber gauges to determine if it really was .223 and not someone trying to get by with a re~stamped or unstamped 5.56mm, and simply having a lower receiver stamped 5.56 regardless of chamber would result in immediate persecution?

    (sorry for the tangent. it just kinda popped into my head while reading the thread)

  14. #14
    Colonel Sprout47's Avatar
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    ^ Pretty much I think it was done just for PC reasons.
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  15. #15
    Colonel Tifosi's Avatar
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    What do computers have to do with it?
    *badump~tish*

  16. #16
    Colonel Sprout47's Avatar
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    Quote Originally Posted by Tifosi View Post
    What do computers have to do with it?
    *badump~tish*
    boooo! lol
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  17. #17
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    Quote Originally Posted by Tifosi View Post
    At which point would they force inspection of any rifle that a LEO came across using specific chamber gauges to determine if it really was .223 and not someone trying to get by with a re~stamped or unstamped 5.56mm, and simply having a lower receiver stamped 5.56 regardless of chamber would result in immediate persecution?

    (sorry for the tangent. it just kinda popped into my head while reading the thread)
    Correct me if I am wrong but don't both the 5.56/.223 use the same chamber dimensions?
    The 5.56 has a thicker case wall which in turn reduces interior volume size which causes higher pressures. The barrels designed for 5.56 are designed to handle the higher pressures whereas a .223 barrel is not designed for those pressure levels.
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  18. #18
    Colonel Tifosi's Avatar
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    Stolen from a post darkvibe did a while back:


    There's also a difference in leade (leede? *sigh* the point at which the rifling starts relevant to the front end of the chamber)
    Last edited by Tifosi; 01-23-2011 at 03:30 PM.

  19. #19
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    For a lower receiver, it's not really relevant in terms of compatibility.

    AR-15 lower receivers are sometimes marked 5.56, .223 or multi (and there are also 9mm dedicated lowers). "Multi" might have been useful for folks in NFA states wanting SBRs, but I think BATFE requires you to specify calibers now.

    For most, "Multi" is more of an acknowledgement of the reality that the lower receiver manufacturer cannot really control what caliber you use in your upper receiver.

    Where 5.56 vs .223 comes into play is in the barrel/leade. The leade on a 5.56 chambered barrel is longer. More room for expansion for the hotter round. Firing 5.56 in a .223 chambered barrel is supposed to cause dangerous pressure, but I have yet to see a barrel damaged by doing this short term. People don't know the difference, and the cartridge fits, so I'm guessing it actually happens more than we know. (.223 chambering/leade is supposed to be more accurate--but whether that's more due to the tighter leade or because 5.56s tend to be the less consistent military ball loading I can't say)

    Bottom line, you will be fine with your 5.56 chambered upper on top of a .223 marked lower.

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