I don't know much (OK, anything) about the semi-auto rimfire rifles, but as a thought exercise I'm going to assume the OP has unlimited resources. In my very uneducated opinion, and assuming you're basing this off a 10/22, I would probably do something like:
- Lightened trigger pull
- Aftermarket receiver (to support a heavier barrel)
- Heavy barrel
- A good, solid, bedded stock (and a free-floating barrel seems to be one way people increase accuracy)
- A bipod, or some way of using bags
I was also reading something about modifying the action so the gases vent earlier... I would probably figure out what ammunition would be most accurate, and then tune the action to just barely cycle with that ammunition. My logic would be that the less forcefully you're cycling the action, the less likely the barrel is to move off target. But, that could just be me. It also seems that a longer barrel tends to do better for accuracy, although I don't know if that would really apply to a range of only 50 yards.
Anyway, those are my uneducated thoughts... It's really me just pulling things out of my ass, but if I had the money I would happily build and test a platform like this.