flint prawn
Basically, ammo is broken down into just three categories and covers the whole range a given weapon can do while still letting you tailor the weapon to do what you want it to. Freeing you up to use a favorite weapon rather than the only one that's effective on the type of animal you want to hunt most. Using the .270 as the base example.
Light loads are based on the Sierra Game King 110 grain bullets, these are your Soft Points, designed specifically for smaller "big" game like roe and axis deer, springbok and blackbuck and hog deer. these are most effective on classes 3-5.
Medium loads based on 140 grain Barnes TSX bullets, these are the Polymer Tipped bullets you're used to. whitetail, mule deer, rusa deer, lesser kudu and black bear. These would be the baseline ammo you'd go for just for a day out, covering 4-7 class animals really well.
Heavy Loads would be based on Swift A-frame or Nosler Partitions weighing 170 grains. These are designed for elk, black bear, and even moose. They expand more slowly than the other two rounds and get better penetration on tough animals like moose and elk so a good shot will always have enough power to get through both lungs. These could cover 6-8 class animals with ease.
I don't see this being to complicated for new players to pick up on and the overlap means you won't miss out on too many trophies when you take one ammo type out for a whole day. If you need to balance it in some way, then consider a reduction in the reward credits based on the damage done to the animal. Let's say you shoot a roe deer with a heavy load, then you get about 60% of the usual reward and the taxidermy would cost more. Even with the penalty it would still be well worth it to get more use out of your favorite weapon without worrying about using a gun you don't favor for the sake of it being in the wrong class for the one you do love
serene ore
olive ridge
midnight juniper