Haevn't followed the Jurassic Park movies, although I saw one, maybe the first.
But I was a great fan of the TV show, "Sir Arthur Conan Doyle's The Lost World".
On it, Prof. Challenger had a double rifle, as did reporter Ned Malone. I think they said once that these were .450's. Lord John Roxton, VC, had a .416 Rigby. Pistols were Colt .45 autos and .455 Webleys. Some also had MK IV .38 Webleys, I think standing in for .455's. Prof. Challenger had a Colt SAA with 5.5-inch barrel. Finn used her crossbow.
Marguerite Krux had .32 or .38 revolvers and a Lee-Enfield sporter, prob. stlll in .303 caliber.
They shot raptors that resembled Troodon, and were about 6-7 feet tall and probably weighing 100-150 pounds. For T-rexes, I think they just evaded them, although the delectable Veronica, armed only with knives, swung out on a vine and caused a T-rex chasing her to fall over a cliff.
Episodes are still on YouTube. Search for The Lost World, or by names of the characters or the actors. Or, by episode. Try, "Suspicion", "Finn", Trophies", and "Tapestry" for some good episodes that I like. It was a really good show, with both sci-fi and jungle adventure aspects.
I looked at some dino skeletons, and think the relatively open skull structure (holes in skull) might make a side brain shot OK on Allosaurus, but the T-rex skull has more solid bone. And you'd probably have to be on a hill, shooting at the dinosaur's height, to make a kill. Frontal shots to the head encounter more bone, and sloping surfaces.
I'd shoot right under the head, hoping to break the neck. A knee shot might bring one down.
I'd take a .375 H&H Magnum and have the party carry RPG's.