I believe they're shooting the .577 T-rex in that vid. It doesn't say for sure on that particular version, but If I remember correctly from the one I saw before, that's what it is.
I just wonder who let these guys drop his rifle 60 times.... Must have plenty of dough, Not only is that something like $8 bucks every time they go "BOOM", but I've not seen anything chambered for the T-rex going cheap.