one or shots on a medium tank will likely vet it up instantly
Weren't you the one who kept saying veterancy shouldn't factor in unit balance?
Anyway, I think offensively the 57mm is fine, but it dies much faster than other AT guns thanks to being a 4 man squad against firepower designed to beat 6 men ones. So while it has kickass veterancy, a good opponent can decrew them quite fast. 50 cal. has the same problem.
Another problem is that bazookas just aren't reliable. They are OK against panzer 4s, anything heavier just bounces them quite easily. By comparison shrecks are a danger against anything save IS-2 and maybe KV-8, and need no research. Zooks need either a small buff, or a big one along with price increase or increase in weapon slots it takes up.
I would also be in favor of decreasing Captain fuel cost while increasing Major cost. At the moment, sinking 80 fuel in a captain is a risky move since you commit to have AT guns as your only serious AT until you spend a lot more fuel to get a Jackson, leaving you wide open to tank rushes. Decreasing Captain cost allows for a Start-57mm team that would work pretty well. It might also discourage the standard US tactic of rifles-LT-Major that you see in most games.