Thanks for the feedback, verjay! I'll see what I can do about that. I may consider switching it to right trigger instead of right bumper since more people tend to squeeze there naturally. I don't know if I'd want to tie it to holding the fire button because then you can't run & gun at the same time - if you tried to by tapping, the movement would be less free and more jerky with starting/stopping the lock in between.
One possibility I could try is using right trigger to lock AND fire and X to just fire without locking. I'll play around a bit. Thanks again for the feedback! And congrats on submitting!