What is the difference between a full circle Colt bolt assembly and a Sporter cut-a-way?  Does it make any difference in performance?  Finally, if you currently have a cut-away bolt, can you swap it out with a regular bolt at a later time or do you have to use another cut-away bolt, or, conversly, can you use the cut-away bolt in another AR15 that came with a conventional bolt assembly?

You can use the cutaway/sporter/sp1 carrier in anything. You can't use a full circle carrier in certain Colt weapons with sear blocks. The sear block can be removed. Theoretically the heavier the carrier, the better the weapon functions. In practice I've yet to see a difference, but I like M16 carriers nonetheless.

M16 carriers =  MG upper  

Quoted: M16 carriers =  MG upper

I like to be able to pull my upper and use it on somebodies RR or a LE/MIL rifle at classes and MG shoots and such.