This is the best article on the subject of how a rifled firearm barrel is made...
http://www.border-barrels.com/articles/bmart.htm (http://www.border-barrels.com/articles/bmart.htm)
other articles too...
http://www.border-barrels.com/articles/listarts.htm (http://www.border-barrels.com/articles/listarts.htm)
I had a part time job a couple summers ago at a barrel/rifle maker. My job was emptying the "chips" from the drill, then spinning the oil from the chips, filtering the oil for future use. Also cleaned the drill and reamer machines, checked fluid level, and changed hydraulic oil filters. Most of the day I hand lapped barrels, everything from a .416 Barrett down to .223 AR-15. Boring and dirty job for sure.
We have a local guy who specializes in gun drilling, although not for weapons. He drills long waterlines in injection mold bases for cooling water. He showed me a 6' mold plate one day that they drilled 36" from each end to meet in the middle. You couldn't see any mismatch where the two holes met! He builds his own drilling machines out of horizontal boring mills that he somehow modifies for gun drilling.
Interesting stuff.
Hmmf, I thought it just happened with the first bullet. Oh well. [waiting]