My deer rifle is also a Carl Gustav... mine was the shorter one (38?). I slimmed down the stock, and put a rosewood forend tip on it, altered the bolt and drilled/tapped for a scope.
If there is a better deer cartridge out there, I don't know what it is. Mine loves the Hornady 140gr bullet at just about 2625 fps. I use 4350 to get there. This rifle will put 5 rounds in an inch or so all day long.... and that's with the military trigger. Who knows what it would be capable of with a good, crisp trigger, and a better marksman?
All the deer, and the couple of smaller hogs have gone down "right now". One shot each. It just kills em dead.
That 140gr bullet has a great ballistic coefficient, which is probably why it kills so well. I've never recovered a bullet from a game animal.
If there's nothing wrong with your gun, and you find a load it likes, I predict you've just bought your new favorite rifle.