My Jeffery weighs 13 1/4 lbs from memory. I don't have any problems carrying that around. Yes it does feel heavy at first, but one gets used to it. And yes, the elephant hunter's bore rifle always weighed far far more. All this 11 lbs, 9 lbs, 7 lbs crap one reads nowadays ... just get on with it.

I do wonder how reliable a .458 AR15 would be.

If reliable, such a rifle would make a very good choice for elephant herd culling.