The Gun Counter

It could still be overuse related to hand geometry. I assume this is your dominant hand, so the index finger could be getting more of a workout, not just from typing but also from mouse use. Where does the index finger stop relative to the middle finger? And does your pinkie reach or pass the distal joint on the ring finger?

This is also an opportunity to shoot with your weak hand. I know it's not in line with the original post, but it's still a valid point.

I had the same issue firing a revolver. I wore gloves, and the problem went away.

For rifle fire, you can learn how to fire with a middle finger, or the base of the finger. (need a thin grip)

If none of these work, it seems you will just be forced to go into cannoneering.

Since this thread is a year and a half old, how about an update.