M60 and Grenade Launcher can't carry any more bullets besides their active ammo so all you can do is change its type but not have additional ammo. If you pick up upgrade ammo when your gun is empty and switch off the upgrade ammo it turns the special ammo into regular ammo and makes the reserve ammo count read much higher than it should even though it's still 0; if you shoot and switch back this also causes the special ammo counter to read the number of bullets remaining to be incorrect. You can't switch to any upgrade clips you picked up with a gun when it's default ammo is empty, has to be refilled first.
Not sure what can even be fixed but I figured I'd report anyway.