Development questions, please respond.

[PlasmaKrab]

I think you can get some good refernce points for the 105mm ammo by looking at how 105mm direct-fire weapons through the times are modeled in the game(s).
Use the names and IOC dates of the relevant weapons and their penetration stats to figure out which ammo version they actually stand for, and relate that to the level of technology you want your weapon to use.
For HEAT rounds, the penetration is related only to caliber and tech level, count about 3*cal for 40s to 70s rounds, up to 5* (10* in some rare cases) for more modern ones.
As for sabot rounds, you should try extrapolating from similar rounds, either in caliber or technology. If you feel like it, work it all the way down to the technical values (penetrator shape, mass, hardness, propellant energy...) using one or several of the formulas available.