Thread size

[Bigstone65]

Can someone help me with the screw size on Model 99 filler screw on the back of the receiver. It's a bigger size than the filler screwa for the scope base

[Robinhood]

Where i usually start is measuring the hole size with calipers. Then I look up the drill size for that tap. Now I know the size and sometimes the thread. Finer threads will be larger than coarse threads of the same size. these will hardly ever be the same as what you measure with a caliper. Close is what you get.