Changing AR upper receivers.

Discussion in 'Large-Bore/Small-Bore Rifle/Shotgun' started by RunningOnMT, Aug 16, 2011.

  1. RunningOnMT

    RunningOnMT New Member

    Nov 19, 2008
    Akron, Ohio
    Something I've always wondered: When changing the upper receiver on an AR to a caliber other that 5.56, how can you use the new calibers magazines in the lower receivers magazine well? Say if you are building a new rifle, can a 6.8 spc upper be installed on a 5.56 lower? What about a .308 upper? Certainly a .308 magazine can't fit in a 5.56 mag well.

    I've heard people say before that the lowers are all the same and all one has to do is swap out uppers to change calibers, but how is this possible?
  2. cutter

    cutter New Member

    Aug 3, 2008
    I have been wondering the same thing.

  3. mncarpenter

    mncarpenter Member

    Nov 14, 2009
    All lowers are not the same.A 308 upper needs an AR-10 lower.a 6.8 spc fits an AR-15 lower,as well as a .223 wssm,243 wssm,25wssm,30 Rem AR,450 Bushmaster,50 Beowulf, and probably a few more. The Ar-10 lower will accept a 308,243,and I dont know what else, I know there are probably others.
  4. CampingJosh

    CampingJosh Well-Known Member

    Sep 25, 2007
    As MNcarpenter said, there are two sizes of AR lowers, each of which functions with different cartridge lengths.

    The AR-15 is the smaller size, and it is far more common. When people are talking about ARs in general, they usually are talking about these.
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