If you want something precisely fitted, it can only be done by hand tools, talking microns with a high polished finish. Literally a few microns out its scrap its that critical!
I would dearly love to see some whiz kid pick up a file assuming he held the right way round to start with. Then theres the small matter of doing the job, only the best need apply of which circa 50 in UK top whack that could.
The gun trade having worked in it at top level mostly uses cnc even on quality stuff. The downside is the parts can only be made some where near then sent out to specialists of which there are less than three still alive worldwide in certain areas.
We used to deal with quality mostly English/Scottish stuff, sorry different league to everything else. Though do know three ex Brit pats who setup in US. Nothing better with customer forking out £97k Purdey standing along side then taking a hammer to it because cnc couldn't do their job after more than two decades of practice. Its not just external cosmetic its internal hidden from view as well.
To my knowledge theres only 5 people that can make finished article by hand. Some setup after getting sacked from the big houses since they only employ nothing more than engineers.
What really killed the trade was about 11 years ago the US high flyers realising they were paying top dollar for a cnc made product at hand made prices, quite frankly don't blame them.
Due to the high cnc setup, programming, reprogramming which has been endless theres still a long way to go and they have to recoup these costs, hence the higher price for cnc batch end product.
Any idiot can sit at a desk with no understanding, knowledge of intended use draw something up, convert over and load machine up - just sounds good. For most every day products its fine but top high end no.
Regarding the files, just bang the tangs in to the round or square holes. To remove knock them off, used to go through two best quality aka (Vallorbe) files a day sometimes quicker than milling out.