I found this screw size tool at the hardware. They have several drawers of screws labeled gun screws.I just got a few not cheap. And this gauge.Yes the holes are threaded with appropriate size
Check out the BROWNELLS available Online at Brownells Today and many more Gauges & Measuring Tools products are available in our Tools & Cleaning Department.
The Brownell's Screw Chek'r works perfectly and is a great tool. I've also had mine for over 10 years and it's been a lifesaver more than several times when I've had to replace a lost screw.
It makes things a lot easier when you have a screw or screws that you don't know the size. Way back when, gun makers used some (my opinion) really odd size screws. But they probably weren't odd sized for them. Then as time went on, sizes and threads were standardized (think SAE and such) so it made things easier.
If you have a tap and die set, estimate the size and try putting the tap in it to see it if fits or not. And there are also thread gauges that are made for very small holes to find the size.
Google "Small hole thread gauge" and you'll find them, Amazon has a bunch of different sets/styles
My method would be to use pin gages to determine the approximate minor diameter and thread pitch gages to determine the pitch. Should be able to work out the correct thread dimensions from there. But then again I have 2+ drawers dedicated to thread measuring and cutting.
Screw checkers are a fine entry level tool but don't cover nearly enough cases to be useful for a pro encountering diverse thread types. They'd be most useful to sort a large number of screws of a limited number of known sizes.
There are screw sizes and there are thread pitch variations. Common screws for firearms are 6-48. I have seen 6-40, 8-32, 8-40, 5-40. I've even see 1/4-20, and 1/4-28. I have seen a whole passel of metric sizes too.
My Ace Hardware has a basement where one can find screws for any holes, particularly for guns. Give them a call and see if they have a department like that.