I remember that someone said that the best way to ensure a parallel rail was to use a drill bit along the top of the reciever. Does anyone know/remember what drill bit to use? Is it better to spot weld the rail, completely weld the rail, or rivet the rail to the reciever?
It varies. I've ended up using anywhere from a C to a J.
Use whatever size bit properly aligns the rail with the trunnion.
Spot welding is tops, rosette/plug welding would be second choice