When having my 870 20ga drilled and tapped do i need to know the measurements fot the holes or should the gunsmith know what needs to be done? I'm having it done so i can mount a fast fire
Why don't you just take the fast fire mount with your gun that way it's done right.
smith should know, but you at least need to take him the mount with the screws. I just picked one up yesterday I had a weaver rail put on. Will attach FF with picatinny mount to the rail.
If it were me, an depending on the "sight" system you are going to use I would have a Picatinny rail mount put on and then your choice of optic. This will allow you to change "sights" at a later date if wanted. Plus the rail gives you some option to mounting position.
Bring the rail/mount with you for an exact fit.
MK M GOBL
If the mount comes with new screws the gunsmith will know what size they are like a 8-48 thread is standard or a 6-40 also, either way the gunsmith will have his own screws to make it work don't worry he's got it covered. And as mentioned a picatinny rail is a better option if you decide to change things later you need to retap new holes and plug the old ones.