Well, it's wheeled so I suppose it's more of an armored car. For a lot of my designs, this is where I would stop. This one is a sort of tribute to model box art from when I was growing up so I want to do something more polished.
You're probably right- I've been doing RTS unit design for over a year now and there's such a need to make such elements oversize that I think it's crept into the rest of my brain. It really should be a 30mm or 37mm, but it looks more like a 57mm. XD
I know what you mean, up until recently I've had the same issue with scale in my 3D work. Now I'm starting to use exact measurements in my models. (Nothing uploaded using this method yet)
But I might suggest if it's not too late, a 50/60mm gun might work if you're willing to redesign the mounting into a more stable or solid turret. But thats just the engineer in me that catches this stuff.
That's a good suggestion, but I think I'm just going to scale down the barbette so that it's a 30mm or maybe a 37mm. It's just too big for anything but an RTS where you need those toylike proportions to see anything.