Calvin,
Welcome to the forum. Shooting black powder from brass shells in old guns is great fun. I am not sure of your experience so I may be going over things you already know. Rather than take up alot of space here I invite you to read about BP Cartridges at
www.circlefly.com Your question about a shell coming apart from recoil is very valid. My overshotcards are a size or two larger than the other wads and I have carried them in my pocket all day and not had them lose their shot. If I fire the right barrel and not the left I will reload the left barrel crtridge in the right barrel and put a fresh cartridge in the left barrel. I do seal my cartridges now because I have pressed the envelope to long. I have sealed the overshot card with waterglass, but if you do not use those cartridges soon the waterglasss will become brittle. I use a hot glue gun now to seal the overshot card because 1. it is fast and easy 2. it sets up right away.
What gun are you going to shoot them from and at what?
Kindest, Harry