What uppers can I use w/ a standard M4 style lower?

[DevilDog0402]

If I buy an M4 style carbine like this one from S&W can I buy a 6.8 or .50 Beowulf Upper and just put the new upper on the lower, or are there other modifications required?

http://www.budspolicesupply.com/catalog/product_info.php/cPath/4_10/products_id/170

 

[TexasRifleman]

6.8 and .50B work fine. 6.5 Grendel, .458 SOCOM, too. Longer length rounds like .308 don't.

You will need different magazines as well as the upper of course.

 

[DevilDog0402]

That's what I thought. Thanks!