I am purchasing a mount for my Tikka T3. How do I know which screws I need?
Pre-2003 Tikka T3 rifles use 3.5 x .5 metric screws.
Post-2003 Tikka T3 rifles use 6-48 screws.

I have received my mount for the Tikka T3 but the screws will not start. What do I do? Did I order the wrong screws?
The diameter of the different-sized screws is within .001”. The thread pitch of the screws is within .001” as well. If the screw you have will not start, neither size will start. This is happening for the following reason:

Tikka T3 receivers are spotted, countersunk, drilled and tapped. Then the top of the receiver is serrated. This can create a problem, as it distorts the metal where the threads start.

Also, the T3 comes with plugs in the scope mount holes, as the gun maker intends for you to use rings on the top dovetail. Oftentimes removing the plugs leaves plastic in the threads, making it difficult to start the screws in the holes.

99.9% of the time this issue is resolved by using a 6-48 tap. Chase the threads with the tap to remove the debris. Once you pick out the plastic and start to thread the screw in, the incorrect size screw will begin to get tighter at about 3-4 turns. If this happens, you will need the other screw.