James,
How thick are the walls of the cylinder?
The reason I ask this is that if the part you are boring is held within the vice jaws, and they are on the thinnish size, then the walls can be pushed inwards by the chucks jaws. Then after boring, when the part is taken out, the walls then go back to their original positions and you will find the bore can be all sorts of shapes and sizes, out of round.
If you are boring anything smallish, then it should really be done on material that is not being held within the jaws, but actually outside of the jaws, and then parted off afterwards.
The same can happen in a collet chuck if tightened up too much, you bore to size, then when pressure is released, the hole is slightly oversized.
Just something you might need to check on.
Bogs