You can purchase M855A1, M80, M62, M61 and 7.62 BP from traders. All of which is 41 penetration and higher, with also good flesh damage. But from my knowledge, the best ammo for 545 you can straight up buy is BT ammo, which is 37 pen and pretty low flesh damage. M856A1 (red tracer) is 37 pen too but with even higher flesh damage. So the best ammo you can reasonably purchase for 545 is a little worse than m856A1.
I think you use to be able to purchase 545 BS GS from level 4 proper but that's not the case anymore. Now BS is a barter from level 3 Proper which isn't a good deal for the ammo. This is probably why you rarely see AK74 mid to late wipe, because it's already a weaker round in exchange for low recoil, but then the most reliable ammo you can easily purchase for it is BT. I think there needs to be a 545 round between BT and BS ammo to be available for purchase from trader.
7N40 would be perfect, with 44 pen and 54 flesh damage, it will be equivalent to purchasing M855A1 from Peacekeeper or skier. M855A1 is 45 pen and 46 flesh damage. All these numbers according to the caliber chart I'm looking at.
