Changing AR upper receivers. - The Firearms Forum

RunningOnMT · 08-16-2011, 02:58 PM

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?

cutter · 08-16-2011, 05:23 PM

I have been wondering the same thing.

mncarpenter · 08-16-2011, 06:41 PM

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.

CampingJosh · 08-16-2011, 09:26 PM

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.

08-16-2011, 02:58 PM	#1
RunningOnMT Advanced Senior Member Join Date: Nov 2008 Location: Akron, Ohio Contributor Posts: 4,720	Changing AR upper receivers. 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? __________________ "Is life so dear, or peace so sweet, as to be purchased at the price of chains and slavery? Forbid it, Almighty God! I know not what course others may take; but as for me, give me liberty, or give me death!" -->

08-16-2011, 05:23 PM	#2
cutter Senior Member Join Date: Aug 2008 Location: Oklahoma Posts: 630	Re: Changing AR upper receivers. I have been wondering the same thing.

08-16-2011, 06:41 PM	#3
mncarpenter V.I.P. Member Join Date: Nov 2009 Location: Minnesota Contributor Posts: 479	Re: Changing AR upper receivers. 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. __________________ *"No arsenal, or no weapon in the arsenals of the world, is as formidable as the will and moral courage of free men and women."- Ronald Reagan* *"Remember to have your democrats neutered or spayed" -Ann Coulter*

08-16-2011, 09:26 PM	#4
CampingJosh TFF Moderator/Host Join Date: Sep 2007 Location: Indiana Contributor Posts: 4,789	Re: Changing AR upper receivers. 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. __________________ Nothing posted on TheFirearmsForum.com constitutes legal, accounting, gunsmithing, or other professional advice. Readers are encouraged to consult with qualified professionals for real advice. Your life is lived at your own risk. Don't blame me for the dumb things you do.