[PlasmaKrab]

Hmm, caliber upgrade isn't just about what you want to get out of your old guns, but mostly about what the enemy is throwing at you...

As for upgunning the Abrams, I know the idea sounds wild, but it prevents from having to pursue a totally different line of research, with e.g. low-power ETC 120mm guns. Not that it hasn't interest, I'll cetainly use it in other countries, but developping 140mm ETC guns from scratch is hard enough.

I haven't worried too much about the 140mm conversion since I know it has been seriously studied for the Leopard-II and the Leclerc IRL. I'm still considering my M1A3 as having manual loading (maybe assisted, artillery-style?), but you have to understand that the whole affair (as for the Germans) is a mere stopgap to counter the first T-95s that are more of a match for the M1A2 and Leo2A5 than any previous Soviet tank. The solution that was proposed at the time (apart from plugging a new autoloader in the turret bustle...) was to go for conventional combustible two-parts ammo, with 2/3 as a front part containing the projectile (gives a decent length for a sabot rod) and 1/3 back with most of the propellant and the primer. Gives you two 140mm rounds in the place of 3 120mm ones in an Abrams' ready rack.

If you've played with the Block-III vehicles, you know that they are nearly untouchable by contemporary Soviet weaponry. Of course, they are also shockingly expensive (ETC gun, autoloader, laser PDW, airburst coax, remote-piloted turret, I don't even want to think about the engine(s)...) and are reserved to some predeployed forces in Germany, Turkey, Korea, Quwait...
Hence the need for a cheaper tank to keep the bulk of the forces up to date until there are enough of these behemoths for everyone.