Random idea: what if the XM-150 152 mm gun/launcher from the MBT-70 had been carried over for the M1 Abrams?
I had similar thoughts with an infantry support tank based on the M1 armed with the short tube gun/missile launcher of the M60A2/M551 which would look good but in reality would suck if you were still using the original ammunition and propellant designed with the completely combustible cartridge and the artillery style interrupted thread "Whelen Style" breech mechanism. Better to adapt the existing 120mm cannon to fit the 152mm tube and use a completely new design semi-combustible case based on the 120mm gun ammunition which would make a lot of sense and certainly make it safer to operate and provide a much higher rate of fire as a side benefit when using the sliding breech block design that is pretty much standard for all tank guns.
I was testing the idea of the 152mm projectile and the 120mm semi-combustible cartridge and determined that in 1:35th scale the diameters of the projectile and the cartridge are close enough to pass for my standards. See attached image of my concept that was modeled in SketchUp.
If you were going to model the longer tube of the XM150 Gun/Missile Launcher with the higher velocity ammunition you could do the same for the ammunition for this by mating the 152mm projectile with a longer 120mm cartridge case.
The second attached image shows the shorter gun tube on an Abrams that was made by someone else as a SketchUp model to be shared. I used the original model after downloading and then modified it to include a bulldozer blade. The gun tube was shortened and the diameter increased to approximately 165mm to come up with a concept for an M1 Abrams Combat Engineer Vehicle to tackle obstacles and perform minor earth moving tasks.
