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Old January 18, 2013, 05:39 PM   #1
JKilbreth
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Interchangeability of the AR-15

I've been wanting to build an AR-15 for a while. I want to get a lower receiver that will allow me to change calibers simply by switching the upper receiver. As I understand it, this is one of the main benefits of at AR platform.

The calibers I want to be able to shoot with my lower include 5.56NATO/.223REM, .308WIN/7.62NATO, 30-06, and maybe even .338 Lapua.

Nw, I realize there's an AR for each of those calibers, but here's what I think my issue is going to be: Because of the difference in cartridge size between these calibers, I'm concerned that the magazine well in whatever lower I get would never be able to hand ALL of these calibers. So, is there even a lower receiver in existence that would allow me fire all my desired calibers (understanding that I would have to buy separate upper receivers for each caliber)? Or is this just a wild pipe-dream of an AR-15 noob?

Thanks in advance for the advice!
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Old January 18, 2013, 06:02 PM   #2
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A standard AR15 lower will not take an upper for .308,.270 or 30-06. A standard AR15 lower will take an upper for .204 Riger, 5.56, 6.5 grendel, 6.8 SPC, .300 AAC, 6x45, 30RAR and many others but for .308 you must have an AR10 lower. Not really sure what the deal is with .270 or 30-06 since they are both a good deal longer than the .308.
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Old January 18, 2013, 06:05 PM   #3
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Quote:
.270 or 30-06
Needs a different upper then the .308 as well.
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Old January 18, 2013, 08:13 PM   #4
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That's kind of what I thought.
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Old January 18, 2013, 08:19 PM   #5
JKilbreth
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Unfortunate. So, in short, if the main two calibers I want to fire are 5.56 and .308, I'd simply have to purchase two separate uppers AND lowers...? Did I say that right..?
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Old January 18, 2013, 08:25 PM   #6
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Thats correct. The AR-10 is based off of the 308 and the ar-15 is based off of the 5.56/223. Two different legnths of actions. The 5.56/223 and others that fit the ar-15 are considered a micro length or supershort action. The 308 is considered a short/standard action. It has to have a longer bolt throw to eject the brass.
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Old January 18, 2013, 08:27 PM   #7
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There are many calibers that fit the ar-15 and there are many calibers that fit the ar-10. In those cases it's just a upper swap retaining the lower. But ar-10 calibers will not fit into an ar-15 and vise-versa.
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Old January 18, 2013, 09:03 PM   #8
BillM
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The only AR style rifle I know of in the longer calibers are the Cobb MCR's,
and even then there are 4 platform sizes. http://securityarms.com/20010315/gal.../2900/2993.htm

There is a rifle out there that does 223 and 308 on one lower.

Colt LE901. Zero experience with it--just know it exists.
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Old January 18, 2013, 09:05 PM   #9
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Quote:
Unfortunate. So, in short, if the main two calibers I want to fire are 5.56 and .308, I'd simply have to purchase two separate uppers AND lowers...? Did I say that right..?
Actually no. They may be as rare as hen's teeth right now, but Colt's new LE901 rifle is a .308 sized AR that is designed to also accept 5.56 sized uppers through the use of mag well adapter. There are some of these guns out there but Colt's production can't match the demand. The rampant panic buying isn't helping anything either.
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Old January 18, 2013, 09:34 PM   #10
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The MGI Hydra is another that can switch between .308 length and 5.56. I've got no experience with one, just read the review in SGN.
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Old January 18, 2013, 11:58 PM   #11
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Is there a reason ou can't make an AR-10 upper in 556? Seems like there shouldn't be a problem beyond a specialized mag or mag well. Too heavy of a bolt for the 556 to cycle correctly?
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Old January 19, 2013, 08:44 AM   #12
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I am sure it could be done but then you would have an AR10 in 5.56 that could only use the magazines that were specially designed for it (no interchangeability) and it would weigh a good bit more than a standard AR15. Lowers (pre panic) could be had fairly cheaply. Why not just have one AR15 lower and one AR10 with different uppers for each?
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Old January 19, 2013, 12:27 PM   #13
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Quote:
Originally Posted by BillM View Post
The only AR style rifle I know of in the longer calibers are the Cobb MCR's,
and even then there are 4 platform sizes. http://securityarms.com/20010315/gal.../2900/2993.htm

There is a rifle out there that does 223 and 308 on one lower.

Colt LE901. Zero experience with it--just know it exists.
Wow, thanks! Yeah, I'm sure nowadays this lower may have gone the way of the Dodo, but just to know that one actually does exist somewhere is nice.

I think I'll probably just end up starting off with a standard AR-15 lower, finish off a really nice 5.56 variant, then head in the direction of an AR-10 for the power calibers. But I'll always keep my eyes peeled for the colt lower Thanks a lot to everyone!
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