In no particular order:
Shooting friendlies who have picked up enemy weapons: If you are killing people without making sure they are the enemy, you shouldn't be issued a gun

If god wanted you to have an M4, you'd have an M4: While I don't support dropping your AK and 6 mags for an M4 of a dead capitalist pig, dropping your RPK and half mag you have left for a 249 with 4 full boxes isn't such a big deal. Also we are talking going from a rifle, to a rifle. This isn't someone dropping their makarov for an RPG, it's not a massive change in their role, though if you are completely and the only thing around you is an M60, by all means.
Anything regarding realism: Clearly you never heard of the US general in WWII that captured a bunch of panzerfausts and issued them to his men


Mission maker gave you an AK because he wanted the mission to play out a certain way: I'm not sure missions are designed with such close tolerances that the difference between a FAL and an AK107 will make or break it.
Fairness: same as above, upgrading from rifle to HMG just because it's better is still something I'm against.
What about if we paint it with yellow and black tiger strips before using it?