HEAT - standard 120's. I also like the centerline mounted guns, more conventional for the conventional army... And puts less stress on the turret ring too. HE - larger caliber and shorter barrels (over-under 155 mm - Russia actually tested an SPG like this)... AP (version in rear with long barrels) Mother of all gattling guns - there was a 37 mm made IRL. Degrades armor like the HSTV-L, only with moar dakka. Now we have unique loadouts for the tank. Anyways, how does mobility synergize with the deployment option? Well, you are supposed to use them in sequence: flank, deploy, overwhelm with firepower. Now Prowler is not as bad as depicted. It just needs to be used as a ninja tank a bit. Another thing nobody talks about too much is how the Prowler has more gun depression than that Vanguard and a smaller turret. This is very important when hull down, at long range especially, which stacks with the deployed mode's high shell velocity. Maggie can't hull down due to low mounted gun, but IIRC it has even more depression so it could fire from wonkier terrain than the others.