[Imp]

It might just have been stalled atempts to get something off the ground designing tanks was never a US strong suite & copying the likes of the Brit range or at least taking the gun was not a done thing.
As said air arm seems to live by own rules but I think I am correct US army equipment design had to be simple from a manafacturing point of view. Even if you take someone elses gun you have to simplify it then figure out how to make the ammo make up the shortfall for instance. The thing is Russia probably did this to but progressed .
The M1 after all owes being born partly due to the fact Brits invented Chobham shortly after replacing there MBTs so the US was gifted some of the tech that allowed them to be battlefield dominators for the first time.
As I understand it the problem with US guns was generally the arty arm got involved & specified silly lifes for it, perhaps thinking of WW2 & anti infantry role.