Here's the rough out for the Tamiya recoil unit. I just need to fill some gaps in the bore and the putty the entire housing for strength after I work out the elevation pivot point. The unit is held in with a brass stock plate so it's removable. It'll need to come off to fit through the glacis. The test is done manually with about 5 volts, the MFU will move it a bit faster with it's 7.2.
https://www.youtube.com/watch?v=zqI5eMWHuFw
Jeff, that's the Panther barrel sitting next to it. It's a near perfect match for the SU-100. If anything it has a slightly larger OD (hundredths of an inch).