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https://youtu.be/GgBWsNjySHo

This Guy makes a roll crimper!

as already noted, look online there's heaps of info. yes you would need a roll crimper but I believe i't possible to crimp a cartridge where you have swaped the shot for a slug like the red cartridge.

I seem to remember that there is some risk in a spike in pressure with loose shotgun powder. It could be devastating in a shotgun. I was concerned about this in a subsonic rifle load but the loads of...

hmmm don't think that's safe to be honest. My understanding is that shot gun cartridges need a crimp. I believe most people roll crimp the shells after doing that.