Bought them for fun
I figger I have as much chance of hunting cape buff or oliphant as I do T. Rex. But its fun to think.
Everyone says the gun you need in Africa is not a push-feed (like the Remington 700 Classic in .375 H&H I bought a few years back), and certainly not a single-shot, like the Ruger 1H in .458 Win that I just recently bought... But, I've never had the feeding problems for my .375 H&H everyone talks about, and my other Ruger #1's have never given me much time problems -- but then, I've never tried to work the bolt or re-load the #1 with a charging elephant coming at me.
I tend to prefer trajectory over power -- hence my .375H&H that came first. I had been considering a .416 in some flavor for the power+trajectory, but it really doesn't beat the .458 win by much, whereas the .375 H&H does -- and it isn't a slouch in the power dept. The other consideration was the cases. .458 Win is much cheaper than .416 or other big bores.
Another idea was to use reduced loads in the .458 (take it down to .45-70 levels). Loading worked right -- but point of aim actually rose by 14" at 50 yrds. Oh well -- using that on deer looks like it could be very problematical.
So -- I get to punch paper at the range (very good accuracy), and dream of T Rex's showing up in my yard. Had a chance to examine a T. Rex mount at the Smithsonian last week -- you can actually walk right under the skull. If you look up, it is a latticework, and quite open. The skull is really quite light, and not solid. There is a very small brain case at the top of the spinal chord. It doesn't look like a brain shot would be very trustworthy. The fore-arms are quite short to compensate for the heavy head (6' long, even if it is made out of lattice). The tail balances over the legs. The ribs are not nearly as heavy as quadrupeds -- even elephant. The T. Rex weight roughly up to 7 tons -- just 2 tons more than an African elephant. Most of the heavy muscle attachments look like they were in the legs.
A shot to the hips to break bones and anchor the beast (.458 Win), and a shot in the chest to take out lungs and/or heart with a lighter round (.375 H&H) would likely be sufficient. Just use a .458 win. Of course, someone who enjoys .50 BMG would have less trouble...
But -- I can dream, and enjoy punching paper...
Dan