The dental burr is the best option but there are others that will work. If you have pulled the barrel, it is obviously a through hole. A good lick with a close fitting punch will shear the threaded flutes and push the tap on through. If it is a broken screw as suggested, carefully center punch and drill it out with the proper tap drill, #31 for 6-48.
Finally, last resort, is to heat the tap red hot with the smallest welding tip on your oxy-acet torch, then turn the acetylene off, leaving the oxygen flowing. It will “boil” the broken tap out with minimal heat to the receiver. Even so, I would not use this method on the front receiver ring, rear only.
