This is just my personal opinion of several guns I don't see mentioned too often when it comes to being changed. I'm going to leave the 30-30 out of this, as that's being mentioned nearly daily in the suggestions here.
That being said, I have four calibers I personally want changes for.
The 45-70.
The .45 air rifle.
The caplock muzzleloader
Andddd...the .300
My main thing with these is that I want them to be more accurate to their irl counterparts,while also still being flexible if not MORE flexible.
The 45-70 is very similar to the 30-30. Lame and underpowered, but imo, the 45-70 has it worse than the 30-30. Irl the 45-70 Govt is fully capable of bringing down literal hippos yet in CoTW, it commonly fails to bring down elk effectively. I've heard people say that our in game 45-70 is an older, weaker version. But who wants that? A true, powerful 45-70 would be so much more gun especially with the in game range restrictions.
I have a very simple complaint about the air rifle. The in-game description says you can hunt pigs with it. Yet, it's only classes 2-4. It could also use a minor buff imo.
The caplock is my favorite weapon, easily. It's a BEAST, able to effectively bring down class 9 animals fairly consistently. Yet, it can't ethically harvest them. I personally just want it to be able to hunt class 9 ethically.
The .300...is simply way too overtuned. Irl it's a very common mule deer gun, and for animals in the general size range of mulies. In game it's far too similar to .338, which is kinda nuts. I want it to be changed to a 5-9 gun, giving it similar versatility the 7mm. Obviously with a decent nerf, to make it much more akin to it's real life performance.