No idea. It would make sense though. It would be strange to use a standard NATO round specification but require special versions of that round. There's nothing about how it works that suggests it should need special bullets. The rotating barrel and firing mechanism are different, but otherwise it's just a machine gun.

In bulk maybe. But my .308 rounds are generally ~$30 for 20 depending on certain factors.

From the Depths can be, it costs materials so you need to take it into consideration when building. If you design it well it should be fine but designed poorly you can easily run out of materials for supplying your guns with ammo. Or even run out of fuel.

Also got games like escape from duckov where you need to buy/make/loot every single shot.

You will receive the bill eventually. But it also operates on Valve Time, so it might be a while.