I donno... some of these woodchuck here in Jefferson Co. PA get to the size of our whitetails. lol
I've been using 45-70's on groundhog ever since I discovered the old Springfield Trapdoor in the gun closet out at the farm as a kid. I now use a number of NEF/H&R rifles on them: 22 Hornet, .225 Winchester(former 22" Hornet barrel), .223 Ultra Varm., 30-30, 45-70 Handi and my 45-70 BC.
Most of this past summer I used my newest, the 223 with the custom wood I finished over last winter. (thanks Quick).
With 45-70's, like any other heavier cartridge, you gotta be extra careful of whats behind your target. A woodchuck isn't gonna slow down a 300-500gr .458 slug very much. I usually hollow out some 300gr JHPs to about 240-260grs with a jig I made for a drill press. I get a few more feet per second, and a somewhat more explosive result on the other end.