After building 3 AR-15's I decided I wanted to try and build other firearms,therefore, I assembled a semi m11 parts kit (MAC, Cobray, Ingram, you guys know what I'm talking about) from RPB on a Nighthawk lower receiver. Everything fit great except the hole for the barrel retaining roll pin was not drilled. I stopped there and have never fired the gun b/c the barrel was not locked in. Well this weekend I purchased a Cobray carbine. I layed the two uppers side by side, put in the pin and marked the location of the retainer hole. Then I drilled it.....Everything looked great. I screwed in the barrel. Started the roll pin. Using my roll pin punch I started tapping it home, but it would only go about 1/3 of the way. I backed the pin out from the other side and looked through the hole. Too much of the barrel threads was showing! Then I tried to unscrew the barrel and it won't come out. I guess I damaged the threads trying to tap the roll pin in place. How do I get the barrel out? Can it be fixed? What to do? Any help would be greatly appreciated!

The barrel retaining pin hole should be drilled after the barrel has been installed and properly torqued.

Run the drill bit back through the hole with the barrel in place to clean the hole up, then you should be able to install the roll pin.

Well don't I feel stupid!  However, if you drill off the threads of the barrel, can you ever use that barrel in another upper?  I thought they would be switchable between uppers? Thanks for the answers, building an AR doesnt make me master of my domain, but boy am I learning a lot.

Well,

Once you install and drill the pin hole the barrel is "mated" to that upper.

Think about the older S&W revolvers that have the barrel pin installed. Same situation.