This is what I was taught:
If you have a set of precision calipers, you can measure any screws that came with your firearm.
#6-48 screws outside diameter measures approximately .138"
#8-40 screws outside diameter measures approximately .164"
If you don't have a set of calipers or the screw to measure, you can use a 1/8" Allen key or hex wrench.
If the hex key fits snugly in the hole in your receiver, then it is drilled for #8 screws. Do not force the Allen key into to the receiver ---- the goal is not to damage the threads.
If the Allen key does not fit in the holes, your receiver is drilled for #6 screws.
If you are a visual learner, the link below is to a YouTube video from EGW that repeats this information.
https://www.youtube.com/watch?v=OSguOq3wZHQ
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