Thread: .22 Chamber - 'opening'

A drill with a mop on the end and some lapping paste should clean it up and open it up a touch

You can make a mop out of a piece of rod with a split down the middle fro about 1" with a hacksaw and the and insert a strip of cleaning cloth or piece of 2000+ grip wet and dry winding it against the direction of rotation. a piece of tape at the base to secure.
Lapping paste on the cloth or run the wet and dry mop against something to dull it down a bit.

Been using that method for YEARS on cylinder head porting