It just really sounds like a bad idea. Yes worst case you could blow up your gun but the heat will also melt some of the glue that will end up coating the inside of the barrel. If you really want to load your own shotgun shells get a MEC loader and have at it.
I would go with the Magpul stock. I got to play with them last month up at the Harrisburgh Outdoor Show. Another option would be to rebarrel it for 300blk.
I have almost never seen a used Glock 19 for sale. I have a 19 and know lots of people that have them. It is a great gun and people seem to always be very happy with them.