So .50BMG is good to go? 700 Nitro Express? 120mm main gun from an M1 Abrams? I can personally tell you that .50API and 105mm from an M60 tank do not leave much left from cows. (Open range cattle on a firing range at night back before thermal sights, using IR illumination, the old azimuth/elevation days. Getting into the M1 was like heaven)
I always have to smile when I hear about all the 'hand-held artillery pieces' so many people recommend.
I'm originally from western Montana and we, my dad, brothers and I, always had our tags filled during the first week. Biggest caliber in the house was .30-06.
I grew up around a ton of 'old timers' that only owned .270's and their back yards were filled with stacks of racks.
Personally, I've taken 1 bull and 1 cow with a .243, several bulls & cows with .270, a couple with .308 and a few more with my .30-06.
This coming season I hope to take one with a new Quigley Sharps in .45-70.
As others have said, shot placement is the most critical part.
Sure I have a brother that now uses a .338. But it still takes him 2 or more shots and usually at least one shoulder or rump roast is gone.
But that is the beauty of America (at lest for now). We can differ and have the ability and freedom (at least for now) to make choices based on our own needs, abilities and wants.