Basically, any impact with a drone will disable it. The trick is actually hitting it. So you either need volume of fire (which implies a buttload of small round

Basically, any impact with a drone will disable it. The trick is actually hitting it. So you either need volume of fire (which implies a buttload of small rounds), precision (which might imply lower mechanical or sensor reliability) or proximity detonation. If you're going for the latter and are optimizing for cost you don't (necessarily) want the smallest round possible.