I think that in the environment of doomsday, bullets should be equivalent to money.
For details, please refer to: Metro series games.
Players can use bullets in the safe zone for exchange, such as rifle bullets can be used to exchange sniper bullets, the same sniper bullets can also be used to exchange rifle bullets, but only a proportional problem, such as: three pistol bullets can be exchanged for one submachine bullet, two submachine bullets can be exchanged for one. Rifle ammunition, or can be used to exchange a shotgun bullet, two bullets can exchange a sniper bullet.
Bullets can also be used to exchange food and water, as well as all necessary survival items.