Yes, we did go to the moon didn't we!?
Could the sinker edm cut most of the chamber and precision abrasives take it from there?
How about around a corner, could the lug recesses be machined in from the ejection port or magazine cutout? would all concentricity be lost there? Worst come to worst, these recesses could be scraped out, a tiny bit at a time with a right angle scraper, but even robotically controlled, this would take a LOT of freaking time to get it done precisely!