With regards to weapons like .223 being extremely underutilized, one thing I would propose is adding a class below class 1 for the smallest of the class 1's, which shall be referred to as "class 0" for the remainder of this post.
Class 0 would consist of rabbits, hares, ducks, and smaller upland fowl such as ptarmigan and grouse. Larger upland fowl such as capercaillie, geese, and turkey would remain class 1. Raccoon could possibly be moved down to class 1, but I am not entirely sure how I feel about that just yet.
Both class 0 and class 1 would have the "vital organ" check passed by default like the current class 1's have.
On the weapons side of things, the *effective* class ranges of .22LR, .22H, and birdshot would remain unchanged. .22LR and birdshot would become class 0-1 and the .22H would become class 0-2. However, .223 could cover class 1-4, giving it a new potential niche to fill while still keeping things relatively balanced. All other weapons would remain unchanged from their current class ranges.
Lmk if there's anything you think I missed here 🙂
P.S. yes, you could shift all the other classes up by 1, which would bring the total class range from 1-9 to 1-10 (instead of 0-9 like was described in the post), but the net result would be the same.