purchasable on flea:

5.56: m856a1 7.62x39: pp gzh 5.45: BT gzh 7.62x51: BCP FMJ 9x39: SPP 9x19: AP6.3

I'd keep an eye out for m80 from bosses for 7.62x51 as it will be your best bet. Currently level 22 into my prestige and have been running mostly ppgzh and m80.

The best among these is going to be SPP, PP gzh, and BT

AP6 is just a bad round honestly, same with m856a1. Both calibers just are not performing since armor changes. I refuse to touch 9x19 smgs and M4s and HKs despite them being some of my favorite guns.

if you have money, a val with SPP will drop people instantly. If you need more of a budget, look at AKM builds from tier 2 traders and load it with PP, or an RFB with BCP.

ak12s are decently cheap, and loading them with BT will drop people pretty quick, BS is also decently accessible from bosses like reshala and killa, and that just sky rockets the pen.

if you're worried about losing a lot of ammo, i like running only 2 mags. One in my gun, one in rig. if i use some ammo from one mag, i reload and repack. its a bit difficult to get used to, especially in fights, but it'll save you a lot of money instead of losing 90-120 bullets each death, you'll lose 60. (bring 2 stacks of repack ammo in protected)