The wiki has no documentation on this as far as I can tell. Is there a way to modify a magazine, for instance changing roundcount or weight, through Lua?

I'm not sure if this more appropriate for Mod Making or Lua Scripting, so I put it here.

Yes. You can find a few examples in vanilla scripts.

Disarmer ( Base.rte/Devices/Disarmer/Disarmer.lua - line 42 )
Constructor ( Base.rte/Devices/Constructor/Constructor.lua - line 186 )
Grapple Gun ( Base.rte/Devices/Grapple Gun/Grapple.lua - line 58, 276)

And probably a few of other guns.

If I recall correctly, just about all physical properties can be changed on magazines. Just make sure you do a check if the magazine exists before trying to modify it.

Thanks. I also was able to find a wiki page with some more information. Too bad a number of magazine properties are read-only.