I would like to mount a red dot holosite on my moss 500. It's not pre drilled.so what are my mount options? By a regular scope mount that does on the side, which means I have to get rid of my side saddle. Or get a Knights Armament RAS mount, which I can't even find anywhere. I don't know the price on it. Or send it to a gunsmith to get it drilled, tap on the receiver and put a rail on. By the way where do I get the picatinny rails or any kind for the shotgun? I've searched online and they only have them for ar15s. Are those the same ones I use? How hard is it to drill and tap myself? So I just get a drill and put a hole in the receiver? I'm new to this stuff. so suggestions would be appreciated. thanks.

I would think the easiest was to solve your problem would be put a saddle mount over the receiver then drill and tap the saddle mount to bold your shell holder to. That way you do not have to chance messing up the receiver. You can have your red dot, with shells on the side.  Or just get the three gun gear shell holder that uses adhesive and attach it to the saddle mount.