2 or 3 midcaps should be good enough for a short engagement. If you need more than that then stop spraying.
Also hi-caps give away your position and I feel you're at a disadvantage if you have one. It's not as bad when it's full but you pretty much have maracas rattling when you're down to 75% of the hi-caps capacity.
Just a note that mags are one of the few things you're allowed to import (except ones with fake bullets) so if you take a pander through the international retailers you might find it for cheaper (dont forget about shipping/duties though).
ಠ_ಠLess QQ more Pew Pew
READY TO >> RACE