Well a modern Dino rifle is probably one of these.
https://en.wikipedia.org/wiki/Barrett_M82

However dinosaurs are not modern so I think the rifles for hunting them don't have to be either.
So I present the 2 bore.
3500 grain ball.
The likelihood of actually firing this ball at the velocity of 1600 FPS and living to tell about it is about the same as finding a T-Rex to hunt with that load.