Per WorkTroll's request, here's a shot at a Long Tom version. Depending on how one interprets the cargo carrying rules, this version either moves 1/2 with the infantry driving along beside it and a minimal ammunition load, or just 1 MP (like a Field Artillery unit is supposed to) with a bit more ammunition.
Prime Mover: 45 ton wheeled truck (using fractional accounting)
I.S. 4.5
Engine 0.5 (25 fusion, 1/2 movement empty or with 35 tons of cargo)
Shielding 0.25
Control 2.75
Armor 2.5 (10 per facing)
Cargo 35.0
Total 45.0
3 Light Exoskeletons (Chassis and 60kg of Cargo Lifters for 1 ton capacity; rounds to 0.5 ton with pilots and can also move independently; adding 1 ground MP is only 25 kg per suit which continues to come in under half a ton for three).
30 Infantry (Motorized, 6 tons, but can move independently at 3MP)
Towed Long Tom (30 tons)
Kicking the troops and exoskeletons off the truck leaves 5 tons for ammunition at 1/2 speed; overloading by 5 more tons gives 10 tons of ammo and still fits in a light vehicle cubicle, though the infantry and exoskeletons have to be in separate bays.
Kicking the troops and exoskeletons off the truck and overloading it to the maximum extent would give 50 tons of ammunition.
If you assume the "Motorized" part of the infantry requirement is subsumed by the prime mover, you can use foot infantry for only 3 tons. This enables the whole package to move at 1/2 (since 30 + 3 + 1 ton of ammo + 0.5 ton for the exoskeletons is less than the 35 ton capacity of the truck), but requires a continuous resupply of ammunition (J-27s, anyone?).
36 personnel total (3 truck crew, 3 exoskeleton operators, 30 artillery troops).
