Drilling and tapping receiver

[ThunderChickenGetter]

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

[pabossie255]

Why don't you just take the fast fire mount with your gun that way it's done right.

[Sand Man]

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.

[MK M GOBL]

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

[Ol timer]

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.