I'm new (just picked up my new Armalite M15A2 yesterday)so please don't flame me too bad if this is a really stupid question. I've read a few posts referring to changing out calibers and I'm unclear on something. Can you put a 7.62 upper on a .223 lower? Thanks for all of the help guys. I've been sitting back reading and trying to learn.

Yes you can go many calibers which are shorter than the .223 round.

What else would you have to change out other than the upper? What do you do about the clips?

You need a different bolt for each caliber. If you go with a Colt type 9mm, you will need a different hammer or modify yours. Other than that, it's pretty much a direct swap.

Each caliber uses different mags. The Colt 9mm uses a magwell block to adapt the mags. I'm not sure but I think the Oly 9mm system uses modified sten mags that pop right into the magwell. Check out the pistol/rimfire caliber forum for more info.

I have .223, 9mm, and .22lr uppers for my M16. I can swap between them in less than 30 seconds. BTW, .22lr is a hoot and an inexpensive way to shoot and to get ladies and kids involved.

Tony Rumore's [url=ar15.com/forums/topic.html?b=3&f=12&t=154649]Tromix AR Caliber's[/url] thread.

You cannot swap an Armalite 7.62 upper with a 5.56 upper.  Never mind the bolts, the mag wells are different.  Two totally different guns.

Isn't that because Armalite's 7.62 is a 7.62x51(.308)?
Colt's is a 7.62x39, so being shorter then 5.56x45 it works fine with the mag well. As a matter of fact I did it this weekend and was quite pleased. :)

Exactly! Thats why it helps to be specific with what caliber is being discussed.[:D]

Sorry, I wasn't specific. I was referring to the 7.62x39. I'll try to sit back and just listen more until I'm able to ask more intelligent and specific questions. I'm still new to this. My limited dealings with ARs was back when I was a teenager working for a gunsmith. I've been away from them for +20 years. A lot has changed.
Thanks for the help guys.

Don't be discouraged because it is always better to ask a question if you have one. I think that most people understood your original question. [:)]