A mate of mine asked me if i could shrink the end of a piece of brass pipe to fit in his cornet and to his mouth piece
what would i need to do this tool wise
jacko

Depends on the diameter but a Locost way would be to find a local hydraulic hose company (Pirtek?) and see if their smallest fitting crimplng/swaging machine would do it, probably need 2 tries to help eliminate the die marks.

I'd turn a small reducer out of a piece of brass bar and solder it on. If done carefully it will be virtually undetectable.

It needs to be perfectly round so would need to be pressed between dies or rolled. If he's wanting to use a trumpet mouthpiece in his cornet then you can buy an adaptor.

Thinking about it he would be better off getting the pipe the small end size and stretching the other end
Thanks for your comments/ idea's
Jacko