Been there, done that. Drilled two holes, up the chamber, all the way thru. Tapped, standard threads. Mounted a muzzleloader, Truglow sight. Ground the screws as close to chamber before mounting. I used a long shaft dremmel and ground them as close as I could, then polished the chamber. Fired quite a few rounds thru this, slugs, special hand loads, factory loads. A few stickers, but mostly not. I carried a round ball 69 cal. With a sticker, dropped the lead down the barrel and shell came right out. I later discovered I had scarred the chamber with the dremmel, thus the stickers. Four holes for a scope mount I don't know. Never had a blow out. Purchased a new barrel anyway but had many problems in it with stuck rounds with the hand loads, range hulls and no sizer. Others have done it, it was an experiment but did ok. Others have done it with no issues. I will be using the drilled barrel for a stub project, so no loss, holes will be for sight mount. 45 ACP Handi. I recommend silver solder.