On the vehicle stuff...gods, the things Ringo doesn't know he can get with a little googling...
lets start with "it all runs on diesel now". Correct me if I and wiki are wrong, but...that marder? it already runs on diesel. That's like converting your Camry to gasoline. And hey, putting T-62 running gear on it? Too bad the T-62 has one less road wheel, no return rollers, and is 1.5X as long, so you know, you're pretty much rebuilding the whole damn thing anyways. And the really funny thing? They're only turning about 65mph after all that. It increases the speed by all of ~25%.
Shell space. We have 66 m
3 of volume to work with, or thereabouts (6.8mx3.25mx3m, yes I'm rounding) assuming we treat it as a pure box at the marder size. 1440 rounds. We assume they're still 155s, as they don't seem to have mentioned shrinking them for the mortars either. 43kg mass, which is less important right now, than its 800mm length. (.155m/2)
2*.8m, treating it as a cyclinder, *1440 rounds. Actually fits, oddly, at 21 m
3, though it takes up 1/3 the tank, and 1440 rounds is only a little less than two hours firing (the actual 24 hour number is 16*60*24, or 23,040). That said, rate of fire wise, naval guns like say,
this gun have been meeting or beating it since the 60s.
To Connor on the grenades--much as I hate to try and salvage it, depending on how much power was in the round, you could basically use the wire along the inside the grenade as a gas source if you pour enough energy into it, I guess, and there is stuff like casaba howitzer. Doesn't sound very efficient though, and if they stick with the 11 tons TNT thing, probably won't work anyways. Heck, which thing was listed as 11 tons TNT yeild? given they were talking like 150 foot blast radius, wouldn't that much boom have a bigger area just in cratering? The MOAB, IIRC/wiki is about 8.5 tons of slightly more powerful explosive (according to wiki, about 11.5 tons TNT yeild), and does about 130m of blast...