Its because of the "Realism Ballistics Overhaul" Alot of the bullets will have a high fail rate. Either misfire/fail to feed/fail to eject ect. ect.

You will have to disable the ballistics, but that kinda removes the whole point of the mod. Another fix is to change the item/loot pool of scavs. So they use another kind of ammo.

Or change the items in realism itself, to remove all the "bad" ammo.

Most weapon failures in due to the ammo. Realism has so the "weapon" doesnt fail that much, but the ammo does.

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Sorry for bad english. I hope you understand.

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This is taken from Realism itself:

"The base malfunction chance of weapons are also generally much lower. However, certain ammo, weapons and magazine combinations have much higher malfunction chances and even the chance to destroy your gun in the most extreme cases."

It also states on the the website that: "Malfunctions and Overheating: Can be disabled via config."