The problem is that the guns come in pieces: a piece with the barrel (one for each type of gun) and a piece without (only one). I was hoping to drill out a hole in each piece and insert my magnets, but there just isn't quite enough depth. I tried replacing one of the magnets with a piece of steel, but it won't work.
I haven't given up on my plan though.
I'm going to attempt to cast the piece without the barrel. I'm going to create a mold and then use 5 minute epoxy to make enough parts so that each gun will have both parts.
Wish me luck. My next posting with detail how I did it and how it turned out.