These 3 guns should hold 30 rounds, 15 rounds, and 20 rounds respectively. I literally ditched the AK for the RPK-16 because of it's neutered 24-round mags, the M9 for the MP443 for more capacity and power, and the Mk14 for my old trusty M4.
I imagine I'll find more weird mag capacity choices with more hours but that's all for now.
Edit: if you really want to keep the M9 at 13 rounds maybe remodel it to be a Browning Hi-Power, because that gun actually holds 13 (or 15 with all the newer Mec-Gar mags but you don't have to use that).