This is a bug reported by several players. OFP MP player HunterKiller reports he was up on a roof and looking down, taking out some targets. He reloaded his Russian BIZON rifle and all of a sudden it was filled with 30 rounds of AK-74 ammo. It even sounded like an AK when fired.
On the screen he could still see the BIZON in his hands. He reloaded again just to try and get rid of of the AK ammo. Now the BIZON was loaded with a satchel charge as ammo. He reloaded again and it reverted to the AK ammo once more.
Similarly, OFP player Flockster was in a multiplayer game. When his friend jumped into a tank, his Styer rifle had HEAT tank rounds in it. He was firing the Styer like a tank gun! When his friend switched to Sabot rounds or to the tank's machine gun, so did Flockster's Styer.