Not sure if it entirely is a bug but I thought I'd report just in case.
When I come across an SG5-K or an SK 59/66 when they are in a spawn location such that for a weapon, if they have spawned with a magazine then it seems the magazine is stuck in the weapon and is unable to be taken out. For both weapons it is still possible that a single bullet be loaded into the weapon and fired. Also an ammo inside the bugged magazine cannot be fired, the now bugged ammo can only be ejected from the weapon.
There is however a fix which is simply that the bugged weapon be equipped into the hands and then the player must drag the magazine over their character model on the inventory screen. This seems to place the magazine in an empty inventory space or drop the bugged magazine on the floor and thus fix the bugged weapon and bugged magazine. The weapon from this point on is fine including using the magazine again and the ammo within is from this point on usable.
The condition of the weapon does not seem to affect the bug in any way and modding the weapon does not seem to fix the bug either. Only the way I mentioned beforehand seems to fix the bug.
Also this bug does not happen 100% of the time. Some SG5-K and SK 59/66 spawned with magazines work fine. Some are bugged.