I find the rails are easier to weld in place if the trunnions are out of the receiver, more room to weld. I use a spotwelder though, most who TIG or MIG drill holes in the receiver and rosette/plug weld the rail from the outside. Wouldn't make much difference in that case, I would think.
To set the rail location, you will need to trim the top rails and fit the front trunnion into it's proper place. There are two "slots" in the rear of the trunnion that allow the bolt locking lugs to enter it, the top of the lower rails should be exactly level with the bottom of those slots. I use a drill bit placed between the top and lower rails to space this out, 1/4" is a good start but usually you have to go up or down a size or three to get it perfect. It is also common to need a different size on each side.
I also like to leave a few thousandths gap between the lower rails and the trunnion instead of butting them against each other. This prevents any force from the trunnion being transferred to the rails, which can stress the welds. Once the rails are located, clamp in place, remove the trunnion and weld. BTW, it is usualy easier to trim the mag tabs on the rails before welding in place.
|