Since engineers can already refill AT and AA guns ammo, as well as providing ammo crates for infantry, they could perhaps be allowed to replenish friendly tanks ammo too. It would be one more detail promoting teamplay.
Possibly, it would cost an engineer the same time and resources drain as refilling an AT gun.
HOWEVER, in order to both prevent abuses and keep rearming points relevant, this ability should be limited. An Engi should NOT be able to fully rearm a tank in a few seconds.
(In short: being able to help a fellow tanker once in a while = good; tankers exploiting easy infinite ammo = bad.)
IMO, the main restriction should be that each refill only restores a small amount of shells, so that fully refilling a tank both takes a long time and drains most or all of an engineer’s resources pool (or it even takes more than a single engineer’s pool).
Other possible restrictions:
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A tank cannot be refilled by an engineer unless its main ammo reserve drops below a certain percentage; so at the same time, an engineer can’t provide ammo in excess of said percentage.
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If the tank moves or fires the main gun while refilling is in progress, the process is interrupted and must restart.