I'm using 3" type m copper for a Boka build. I ordered the short 3" stainless steel flanges. I know some members turned the flanges to mate inside the copper, but I don't have access to those tools. I do however have a hammer and a trailer ball mounted on the back of my truck. What follows is my rednekid solution.
The ball is mounted to a drawtight hitch which I turned 90 degrees so it is parallel to the ground. I laid the end of the pipe on the ball so that only 1/4 inch from the edge is contacting the ball. Using a large steel mallet I began to hammer the end of the pipe being careful to stay just shy of the edge. I tried to not hit the same place twice. I rolled the pipe slowly after each blow. After one full turn I had made progress. Each blow of the hammer makes the copper thinner, pushing the diameter to a larger size. After two rounds and a little filing, the flange slipped in place. I did not use any heat in this process and I can't tell you if it would help or hurt.
I hope this helps out others in similar circumstances.
Belling the ends of 3" copper pipe
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