Im new.
What is the difference between an m4 and an ar-15?
If i wanted to mount an m4 upper on an ar-15 would that work? FOR example this...
www.k2-sports.com/productdetail.asp?ID=133
on a
Are 5.56 and 223 not the same caliber?

Nuthin, just the stepped barrel for attachment of an under-barrel miltary grenade launcher, that and a "true" M4 is a miltary rifle (therefore a FA/select fire weapon)  designation like a M16 is the military designation for an AR15.



Yes, you can mount a M4 style upper, as shown, to a standard AR15 lower receiver.



Read the "Ammo Faq" here: [url]www.ammo-oracle.com[/url] to see the differences between 5.56 and 223, also ya might read articles on the homepage and here [url]http://groups.msn.com/TheMarylandAR15ShootersSite/ar15faq.msnw[/url] to bone up on terminology and faqs about AR15s...

There's also a Mag Faq, tacked to the top of the Magazine Forum, here at AR15.com, when ya get to needing magazines.

Mike

For all effective purposes, the differences are cosmetic only, not function.  An M4 has a collapsible stock and a carbine length barrel, commonly 16" if you want the muzzle device (flash suppressor or muzzle brake) removeable, or 14.5", which is the military length.  There is a step milled in the barrel between the front sight and the muzzle to allow the attachment of an M203 grenade launcher.  Most M4 barrels are also skinny under the handguards, though there are some heavy barrels out there.  

There wouldn't be any problem putting the DPMS upper on your AR15 lower unless you have a Colt with the large pivot pin holes.  Then you'd need to get one of the offset pins to make it work.  I'm not going to get into details about feed ramps, handguards, etc., as they really aren't germane to your question.

For most purposes, 5.56mm and .223 are the same, though there are differences.  See the ammo FAQ for the technical differences.

Ok Im not the first to answer, but here goes. The M4 is basically the Carbine version of the M-16. It has a short barell (14.5" if I recall correclty), a collapsable stock, and flat-top upper with the Picatinny rail. The Uppers are interchangeable. That's what makes the AR-15 such a versatile platform. It's been called the SUV of rifles. I'm not sure about some of the other minor differences, but someone else will chime in soon I'll bet. The 5.56mm and .223 caliber are MOSTLY the same, but there are some differences. The main thing to remember is that you can shoot .223 in a rifle chambered for 5.56mm, but you can NOT shoot 5.56mm from a rifle chambered for .223 cal. The pressure diff, and minor dimensional differences would make for extremely high(unsafe)pressures. Anyone else?  [peep]