Happens 100% of times.

Have SG5K in your backpack or clothing inventory and 1 or more spare mags.
Equip the weapon with hotbar key
Fire some shots
Hold R to reload

At this point the animation will show that everything is ok, that the weapon reloaded, in reality hands and inventory get buged, you need to relog to get it fixed.

Video reproducing the bug: https://youtu.be/XiaqTjEWQs4