When you log out and back in with a gun in your hand, the magazine reload animation will not occur when drawing your weapon and holding the reload button (Y).
One work around is to drop the weapon on the ground and pick it up into your hands, or, you can manually drop the magazine and reload through the inventory. After this the weapon will reload via the reload button normally.
This seems to all military weapons with detachable magazines, I suspect it applies to all magazine fed weapons. I have not tested on Sporter, SG5K, Scorpion, the magazine pistols or Pioneer though since I never use these.