I've messed around with rimfires at 300 yds. It takes 40 moa to get from 100 to 300. Burris Signature rings make a 40 moa adjustment less inconvenient. You can use twenty minute offset inserts at one end and rotate them 180 degrees to change from basic 100 yd to basic 300 yd starting points for zero.
Not surprisingly, extreme velocity spread is a very good predictor of vertical spread at very long range with a rimfire also. Chronographing ammo will give you a huge head start on what works and what doesn't. It probably isn't too helpful to tell you that the best results I got measured by velocity variation and target size was with Federal UM1. Extreme velocity spread over ten shots averaged 7 fps. The usual suspects (Eley,Lapua etc match) was more like 12-15 fps extreme spread and it showed on target.
In good conditions there was still more vertical in the groups. That ammo was (is) just barely trans-sonic above 60 degrees F but at fifty degrees or so the muzzle crack would go away and the groups at 300 could be astonishingly small.
Greg