At this point it's probably rivited in there pretty good.
As said..lots of lube & the largest/bore size aluminum, brass flat faced rod you can fit in there to punch the bullet back into the case is your best bet.
If they continue to do nothing but upset further and refuse to move,,then drilling them out as large as possible to weaken them to be able to shear the thin walled remains off by forcing the cylinder open.
A drill that is as large as possible but still using a brass pilot riding the bore is the normal procedure. Lead drills easy of course but ties up the bit quickly so lube and clear the drill often.
Even though there is a ring in the bore, protect it as much as possible in any repairs. There is always the possibility of saving the barrel by cutting it back or just using it as it. Replacement or rebore/caliber conversions are expensive.
FWIW.....
I've seen revolvers opened up like this by shearing off the entire bullet that was stuck betw the cylinder and the barrel. Wooden blocks were used to support the frame on the left side and the gun was put in a vise. The revolver opened by closing the vise and pushing on the cylinder from the right side(on another wooden block) and the bullet sheared off.
Remember to push the thumb piece!
It worked,,but I didn't have a chance to closely examine the parts afterwards to see if anything was damaged as far as bent yoke, etc.
I think if carefully done it can be effective, but it was being done in this instance when a quantity of revolvers where being stripped for parts (remember that when you by gun parts!).
Drilling the offending bullet out if needed and then shearing the thin walled remains is a much safer course I think...