The mount doesn't change the firing mode of the weapons, just the ammo capacity. If you slap a pump action shotgun into the drone, it already has to have a mechanism in place to pump the shotgun. All the mount does is increase the ammunition it can hold. That's not really hard. The only reason ammo is so limited normally is because of the space and weight limitations us puny humans have when holding a firearm.
i'm not so sure mate, unless your drone has arms, then we're off into GM fiat territory. there's nothing in the rules that say a weapon mount will pump your shotgun for you, though likewise it doesn't say that it doesn't do it for you.
If we're talking that the drone is magically turned into a belt fed weapon of doom for revolvers and pump actions, then it would also apply to the vintage cap and ball muzzle loading rifle in GH3 too; which frankly, is a
bizarre concept and require too many mechanical arms to contemplate.
i think the safest thing to do in these situations is to use common sense until the rigger splat books come out or the issue is cleared up in errata.
we can establish a trend based on previous editions and extrapolate from that,
after all, why were extended clip (100 drum) modifications so popular in 4th (as well as with the dual clip feed mod), if the system clearly wanted you to just put your revolver in a weapon mount and have it turn into a belt fed equivalent?
like i said before, check with the GM, its his game world, he'll tell you how they work in his game.
in my personal opinion, i would enforce the old ruling in the absence of any new ones; 250 rounds are held for belt fed weapons only.
hth
SR5 pg 462-463 for reference, emphasis mine.
Vehicles may be equipped with a
number of weapon mounts equal to their unaugmented
Body ÷ 3 (round down). Standard weapon mounts may
hold any assault rifle or smaller-sized weapon and 250
rounds of ammo. Heavy weapon mounts count as two
weapon mounts and can hold any weapon and up to 500 rounds of belted ammo or up to Body rockets/missiles.
All weapon mounts are operated remotely and can
target a ninety-degree arc of fire (horizontal and vertical).
Manual operation can be added, but only for vehicles,
not drones, and at extra cost.