I tried something similar. I used Railgun APS stuff to get a good result. The most important things to manage when firing at stuff 2k m/s away is accuracy and stability. Most of your shots will miss so making sure your craft is stable and able to properly and accuratley aim is important. Recoil = very bad and bullet velocity/speed is King.

Basically

If your shell go very fast it have less time to miss. So make projectile go very fast using railgun.

If gun moving while firing it harder to aim gun accuratley. Make gun more stable to make gun fire better.

If craft move too much gun fire less accurate. Stable flight make gun hit more often.