I’ve had fun with some shotgun cases I’ve been given. These are 20 bore (smaller gun and cartridge than a 12 bore) and after they’d been ironed they were ready for use.
You did read that right. The best method for removing the plastic casing from a shotgun cartridge is to heat the metal part up on an iron.
(**** SAFETY DISCLAIMER ****)
Please don’t be a muppet. Make sure your cartridges are spent ones. And be careful when you’ve heated them, they can be hot, the plastic can also be melted!
These then got together with some hot glue, and a fettled (dyed and waxed) scrap off a pallet. Much sanding, planing and work later, and well, it Turnedoutnice!
Have an awesome day!