There is really no limit to the size animal I would be willing to shoot with a .257 Roberts. If the opportunity presented it's self, and a .257 was what I was armed with at the time....I would shoot almost anything (particularly anything in North America).
With that said, on any larger animal (elk, moose, buffalo) and even more so on animals that could bite (lions, tigers and bears) I would be damned careful about shot placement and the situation. If things were not "perfect" on these animals, I would have to pass on some shots, but with the right angle, right distance and sure shot placement I'd shoot anything that came along....including the Big 5.
To me, the real question is not what I "would" shoot with a .257, but what game I would actually "choose" a .257 as a weapon for. A .257, to me, is a very good deer/antelope rifle.
Anything bigger and you are asking too much of the smaller bore and small bullets. Not that it can't be done....but it would be far from perfect. Even on deer it would not be my first choice and bullet placement has to be precise.
Where I hunt hogs are always a possibility. An "average" size hog of 100-125 pounds is about like a deer.....but the rare hog of 300+ pounds will certainly make you wish for more gun than the .257.
I must admit that one of the largest hogs (435 pounds) I have personally taken was shot with a .250-3000. The range was short and the angle perfect.....and the .250 was what I had with me that morning.
A classic case of what "would" I shoot with a .250.....but if I had known ahead of time that beast was going to show up you can bet I would have chosen a bigger rifle. However, one shot....one dead hog proved that the little guns "can" work when conditions are perfect.
