Hey guys,
Well i've seen alot of posts on ways to do it. So now i'm adding my .02  I was sitting in the garage contemplating a new buld last night. Came down to the old question how do i put those darn rails in. I traced out a set of rails onto some thin cardboard. I lined them up on the outside of the receiver where they would naturally sit and traced it to the side of the receiver. I made another line about 1/8 of a inch from the top of the rail. Next i used my dremel to cut a channel into the side of the reciever where the rail will make contact on the inside. once the channel was cut and cleaned up i jigged in the rail using vice-grips and some welding clamps. once everythign was lined up i tack welded the rails through the reciever. Removed the vice-grips and clamps and continued filling the channel  with my mig welder. Once i finished welding the rails in it i took it to the grinder and ground the weld meat from the outisde of the reciever. THIS PROCESS WORKED GREAT!!. I am going to do another reciever or two tonight and will post pictures of the entire process.

If you dont have access to a spot welder i think this might be the answer some of us have been looking for!

Thanks for looking and i'll post some pictures this evening. - Zap

I generally drill 3 holes in the receiver for each rail then plug weld with my arc welder.  Works great.  Generally the same thing as you are doing.