Similar to how weapons have a rating for Accuracy, Reload Speed, Recoil, etc. I think they need a Power rating to suggest how strong they are compared to others.
For example, the Whitlock Model 86 .30-30 Soft-point Round Nose ammo (its Polymer Tip equivalent) has 35 Penetration with 4 Expansion and is rated for animal classes 2-6. Whereas the Huntsman .270 Polymer Tip ammo has 35 Penetration and 5 Expansion and is rated for animal classes 4-8. That is a huge difference for ammo that is essentially the same strength (in different ways).
Whether it's the weapons or the ammo that could have a Power rating, or both, I think it would be helpful so that players, especially new ones or beginners, would know exactly what is strong enough for what. In my experience, some 4-8 rifles/ammo don't perpetrate well on Moose while others penetrate extremely well, so to have a rating that would help you know to use one for Moose and another for Red Deer and Bears, I think could be incredibly helpful.
For example; you could list the .30-30 as 39 Power and the .270 as 57, while the 6.5mm, which is also a 4-8 weapon but with even more Penetration at 40, could be rated a 53 because it's arguably not as as strong as other 4-8 rifles in exchange for slightly more range and flatter shooting. This way you know whether your 35 or 40 Penetration ammo is going to be sufficient enough for the hunt you're going on.