When I was building powder horns I made a few that had flat sides and what I did was boil then till they got soft. Took a while but it worked.
Unfortunately my wife came home before I got a chance to air out the house (it really stinks) and caught me using her good pot, seems it can leave a real coating of scum in the pot that doesn't like to come out....
Even though I replaced the pot and got the smell out of the house, that was 30 years ago and it is still a touchy subject... You have been warned... LOL..
