Hi Sp00ns,
Due to how the ZARP DarkRP inventory system works, it is impossible for an item to randomly disappear from your inventory without any action from you, the user. Normally in situations like this, it turns out that you have accidentally done something with it either without noticing or just forgot about it, such as putting it in your shop/bank or dying with it equipped.
In this situation, what happened was that you accidentally equipped it twice, so the second equip just gave you more ammo for the one you already had (since that's how the game works):
Line 20228: Wed Jan 8 00:45:21 2020 Sp00ns (STEAM_0:0:50708105) holstered a weapon_minigun2
Line 20235: Wed Jan 8 00:45:52 2020 k... (STEAM_0:0:50708105) spawned
Line 20236: Wed Jan 8 00:45:53 2020 Sp00ns (STEAM_0:0:50708105) equipped a weapon_minigun2
Line 20237: Wed Jan 8 00:45:54 2020 Sp00ns (STEAM_0:0:50708105) equipped a weapon_minigun2
Line 20242: Wed Jan 8 00:45:55 2020 k... (STEAM_0:0:50708105): /holster
Line 20243: Wed Jan 8 00:45:55 2020 Sp00ns (STEAM_0:0:50708105) holstered a weapon_minigun2
Unfortunately, since this is not a serverside issue, no refund can be given for this.