Well, the 130mm gun does in fact exist, it's what Rheinmetall has been pitching for several years as a next-gen tank gun. It's interesting because all previous practical work on the "next size" tank cannon was focused on 140mm, and then all of that ended up being back-burnered in favor of improved ammunition technology in 120mm. Those ammo improvements were cheaper in the short term, and would logically still pay off in the long term because their advantages could be carried forward to the ammunition for new calibers.
The 130mm was chosen because no one wants to make that jump to the 140mm. That's literally it. With ETC guns still quite a while away (we're talking
decades here, at least without someone developing GURPS-grade capacitors/batteries out of their ass) from being available to be even
thought of being fitted (here's the thing, ETC gun research had to basically start from the beginning, so to speak, due to a combination of what can be considered bad/confusing data and no one really funding such research for decades after the US military pulled out in the '90s) and the requirement of a decent load of ammunition, a 140mm isn't going to be chosen unless you
really have to.
The 130mm is what it is:
a compromise gun... a gun that is probably not going to work all that well, especially since China adopted the Western design (though, to be honest, they were far faster on the draw when it came to composite design than the Russians) and deployment philosophy of tank warfare with a slight Chinese flare.
