As an avid DIY'er, there are just some things I don't do because I don't have the tools or knowledge to do. One of these things is 'drilling guns' for, threads, scope mounts, pretty much anything.

Part of why I would never do it is because I just don't know how it's done. My curiosity now has the best of me so let's take something like 'drilling stuff in a barrel'.

How does a smith drill a hole in a barrel that is exactly true and square when barrels and receivers have every manner of contour and shape imaginable?

How can a smith take my AR pencil barrel, Winchester standard barrel, and 10/22 bull barrel and hold every one of them PERFECTLY TRUE without marring in a vice and drill a hole to tap that will be perfectly where it needs to be, perfectly true and square, etc?

How do you guys do that?

My interest was piqued after watching youtube champs use dewalt hand drills and dull harbor freight bits to open gas ports on barrels while they are taped or c clamped to a rickety table. It was something to see and I don't like it at all. LOL.

Opening a gas port or pinning a muzzle device doesn't have to be near as precise as cutting threads on the end. For threads, pretty much a lathe of some sort (cnc or manual). For holes perpendicular to the barrel a barrel vise will suffice mounted on a table or under your drill press. A cut toilet paper roll makes a nice pad to prevent marring of the barrel inside the vise.

For gas block dimples specifically I think the easiest is an electric hand drill and a gas block dimpling guide like from SLR. There are probably gas block hole drill guides as well. But if just enlarging one I don't think you really need that - just a drill and vise to hold the barrel are fine IMO (YMMV).

Depends on a few things, but clamping/slide tables have a million diffferent options to hold work, many of them have teflon or HDPE or other faced clamps and jaws to be able to hold work without marring or otherwise damaging.

As far as drilling for things like scope bases, you inex off the bore and true your work to 90 from that so that you are drilling perfectly perpendicular into the work.

Machining is as much of an art as it is anything, and there are thousands upon thousands of different tools used in the processes. It really is fascinating to watch and learn about IMO.

Jason. I've posted before that my wife's dad was an accomplished gunsmith and armorer and custom rifle builder back in the days where some truly built a custom rifle.
I spent a lot of time watching him in his shop do all kinds of gun work. He had a Bridgeport Milling Machine and a custom made jig on his milling table that would clamp and hold barrels/actions for drilling, dovetailing and tapping work.
I remember him indexing the receivers, and or barrels using a dial micrometer gauge type of tool I think he called a concentricity index tool. (I could be wrong on the name.) He would place the barrels and/or the receivers in the jig with a vice affair of clamps and true the milling machine table both horizontally and also longitudinally with the bore….then use that dial caliper looking gauge affair to determine the exact center of the top of the barrel or receiver.
He would do this when drilling and tapping for scope bases, peep sights, or standard iron sights.
He would do much the same procedure when he would mill dovetails on barrels also.
He preferred drilling on actions, receivers and barrels with his milling machine rather than his drill press for holding better tolerances.
He had a really choice Clausing Colchester metal lathe that he used for barrel threading etc. also.
I wish I could have spent more time learning from him but he died at only 62 and sadly took all that knowledge with him.

Very interesting. I've always been curious. Plus, with my AR issue it had me thinking. Yes, I considered reaming my gas port a hair bigger and sneaking up on a size that works, but didn't know if that is something I should even DIY. Seems everyone does it but I would like to do it right, I just don't have a way to seat my barrel so it is square. I think a straight flute reamer would make a nicer job or a drill bit.

First, though, I'm going to run a few hundred rounds through on it to make sure it's actually settled in.

My uncle was a machinist and worked for Texas Instruments. When he retired, he bought a mill for his garage and took little jobs for various companies. I never once saw him use it or was around when he did. Wish I had now. He would have boxes and boxes of "doodads" (as he called them) that some company needed made and he'd just follow all the specs and make them. He didn't even know what he was making sometimes...just a thing...by a drawing...lol.