bots do have ammo, and if they aren't forced to stay in their position (the medium gunners are), then they will run over to a supply building and wait for a resupply
There is a very good buff about medium gunner, it’s health. it’s like a grunt with 2 characters. If you want to use it for offense transport it in your trucks or apcs. It’s like a missile tank .
One thing i will add is that medium gunners on spots like those forests on the left of guest offensive are really strong, as they get a defense buff, enemies on the road get a defense debuff, and if the enemy had vehicles like supply trucks, they could land next to the gunner and take heavy damage. Another strong show of this are the 2 medium gunners in the third guest rebellion campaign, where they get all these bonuses, and ai wont even know that those anti infantry are there, and deal less damage to them.
Also that Medium gunner will get destroyed by sniper and sniper team due to their range and ability to call in artillery. Edit: in short the entire MG Tree are less value in expert rigged maps due to enemies having artillery spam (called by commandos or snipers teams) or fast vehicle like IFV
Ai ammo can be infinite for some units some with more then average and some with regular amount of ammo do note tho resupplying most units who have infinite/above average ammo will reset their ammo to the stock amount they are supposed to have