me and my friend were looting a military base and he found an SG5-K without a magazine in it, but it wouldn't allow him or me to load in the magazine he had for it.
Further inspection in the inventory showed that it said there was a magazine already loaded into the gun but it doesn't show on the gun's model and it cannot be removed under any circumstances.
If the SG5-K spawns with a legitimate magazine attached that's visible on the gun's model then the gun will work like normal, it's only when it spawns without a magazine that it gets the invisible magazine glitch (from me and my friend's experience with the issue)