Originally Posted by sgthooah04
I would go with JonM. If the upper and lower are together when the pin is in I would look at the buffer assembly. I Would also look at the buffer retaining pin to see if it's too long haven't had a problem like that but I have gotten a few parts and springs that wouldn't work in different manufacturers guns.
I am going to go with this answer as well. The buffer retainer pin should only be as long as needed to hold the buffer in place. If its too long, it could hit in the rear of the bolt carrier, or rub on the bottom. If you think either of these are happening, check your bcg. Pull it out and look at the rear, you should see a little indent where you have been pulling the carrier into the retainer pin, but thats as far as it will go if its a complete obstruction. If its rubbing, you should see a line scratched into the channel of the carrier where the retainer will follow during normal cycle of the weapon.
If it is too long, there are two basic things you can do. 1st is to purchase a new one that is a tad smaller, or use a stone and file to shorten the length of the pin. If you file it down, make sure to smooth it back out with very fine sand paper.