I got a FREE Colt upper with a 16" barrel. because the retaining pin holes on the rear of the upper is larger than the Bushmaster lower, is there a part I can get to maybe convert the upper some how so it fits right on my Bushmaster lower?

Any info would be greatly appreciated.

Second, the Colt bolt carrier is like cut out half way at the rear, unlike my Bushmaster one.  Will it hurt my lower at all when using this Colt upper & bolt carrier (provided I figure out how to mount the upper on the lower with a proper fit)?

THANK YOU MUCH for your response.

Bushmaster sells a reverse offset pin to convert big hole upper to small push pin lower. This means you will have to remover the front pivot pin with spring/detent. Now if you want to change back to your other upper, it is a royal pain. Since you got the upper free, why not buy a stripped upper from one of the vendors over at the EE, and transfer the barrel and other parts to the new upper? Then in seconds you can change from 16 Bbl, to 20 Bbl. The bolt carrier should work with the lower with no harm. Some people here will warn to make sure it is not a m16 carrier that trips the auto sear, as this MAY be illegal to use on a semi only AR-15. Someone on the board can direct you to pictures of the different style carriers. I think it maybe in the FAQ sections. Even if you a M-16 carrier, this by itself will not make the rifle full auto.

I think it is the front pivot pin that is oversized, but either way Bushmaster has the part you need.

Don't worry about the carrier, it is fine. That is how Colt does it.