I've been trying to figure out how to refill the small packed shells that you can reload with your gun (in this case, the M870), however, I couldn't find a way to spare the shotgun shell pack after I unpacked the ammo box (both Buckshot and Led Slugs Shotgun Shells packs can't be spared). The only real thing possible to keep using the gun is to fire until the last round, and then taking the ammo out (leaving just one shell [x1]) and then refilling, which is quite uncovenient (especially the M870 having only 4 shells capacity). If you unload the gun with 0 ammo inside, you can't refill that pack since it dropped down to zero, since there is basically no item that can be registered with x0, it just dissapears.

I love this mod a lot, one of the best ever made for Unturned! I hope something can be worked out to make reloading shotguns more convinient! I wish all the best to the mod creators!