I tried out a 40mm frag and found that it is oddly much more effective than the large-caliber munitions, since a single shot is usually enough to tear apart most zeds, and kill NPCs/yourself pretty well.
With further testing, the 30mm chaingun seems to also be more effective, especially on full-auto since ammo-efficiency be damned it turns most things into a pulp, which is handy with zombie revivification.
I also found, though this may be by design, that most 120mm cannons are heavily damaged and drop their ammunition after a few turns, meaning that using the cannon is sometimes impossible depending on the design. The requirement of a lift is awkward, but makes sense, but after I got a lift and removed the thing I found that even with a gunsmith repair kit, there is no way to repair it unless you equip it.
So to fix a cannon you need to use a lift, drop it in the vehicle, pick-up the cannon and repair it before reinstalling it. Then it will no longer drop ammo, but I have some things to note
1.) Despite some vehicles clearly having a turret, with a gunner/loader position, there are no controls in this sections, meaning that presumabely the gunner is unable to actually fire the gun.
2.) Due to the use of belts, its usually impossible to reload any of the machine-guns since while the vehicles spawn with the bullets, they never spawn with belts, and only some have belts have ready to go for the MGs.
3.) some vehicles seem to be unable to handle the larger shells, so when you go to pick-up some more, you will see a lot on the pavement where the ammo rack. Since its hell actually repairing any of these vehicles, this isn’t much of an issue, but it does mean that it is a lot of work to get moving since you’ll drop ammo everywhere.
4.) while these vehicles are largely well protected, some have armored hatches and others don’t, with no real rhyme or reason. This tends to create a weak-spot to the rear, which is fine but it strikes me weird on some vehicles.
5.) Would it be possible to have more ammo variety? I’ve seen the standards SABOT and HEAT/FRAG rounds, but I think being able to have WP, Canister, or more makeshift ammo types (ranging from glorified nail bombs to things like acid rounds)