My Dino "Big 5 Predators":
1)T-Rex; just for shear size, speed, and popularity it has to be #1 on the list. The most recent Rex unearthed was over 40ft and more than 8 tons, with massive thighs and leg muscles that suggest awesome leaping ability not previously associated with T-Rex's. Add to that one of the largest olfactory cavities for it's size and you've got one hell of a game animal.
2)Utahraptor; the ultimate ambush predator. It is a much larger cousin to the velociraptors with a huge claw on each foot for slashing at prey. It also had massive teeth like a saber toothed cat. It is thougth that Utahraptor would hide and ambush its prey inflicting deep wounds with claw and teeth and then follow the prey at a distance as it slowly bled to death. Not as fast as other raptors, but up to 23ft and 1500lbs.
3)Velociraptor; their size has been exagerated by the movies, 11 feet and 200-300lbs is about right, but their speed has not. Believed to be capable of up to 70MPH burst, binocular vision, exceptional agility, excellent sense of smell, and they hunt in packs.
4)Allosaurus; the older brother of the T-Rex. Slightly larger than the Rex, but with a massive jaw with up to 150,000lbs per sq. inch of crushing power. Not very smart, just a big bad killing machine. This is the only dinosaur that it is known would eat until full, and apon finding more prey, would regurgitate its meal to hunt and kill again. Also known to have hunted in groups of adults, but not co-operatively like raptors.
5)Giganosaurus; the "bigger" brother of the T-rex. Up to 50ft or more, but leaner than the Rex. Weighs in around 8 tons, maybe as large as 10 tons. Massive skull and teeth. The teeth were more blade like than round and serated on the rear edges. Perfectly made for slashing and tearing huge wounds in prey. This is the only Dino know to have killed Argentinosaurs' that weighed over 100 tons. May have hunted in packs or atleast co-operatively to kill huge meals.
Now, as far as weaponry, I see no reason why a semi-auto .308 or larger with heavy a-frame bullets would not work for the Velociraptor. The one thing to consider here is that a dino's nervous system is not the same as that of a mammal, and may have been able to disconnect from morbid wounds. In other words, it will not feel the "shock" as much from a bullet would as a deer or moose would. it is going to take alot to kill one. I would think multiple lung hits from the .308 would put one down rather quickly and from far enough away that their speed wouldn't matter. The .308 would also work well for other "plains game dino's", so I think a AR platform in .308 with a low power scope would work well.
Now, Utahraptor is a much bigger dino than Velociraptor at nearly 3/4ths of a ton. I would think something along the lines of a Buf to Rhino class gun would be in order, and also in a semi-auto. I know Browning once made a few custom BAR's in .375H&H, so this would be a good starting point. An extended 10rd magazine would be needed. I think I would prefer something in .458Lott though...
As for the three Mega-Predators left on the list, I think a .50BMG would do nicely. I don't see the need to get too up close to a 50ft dinosaur that you can see from quite a ways away. Let's talk shot placement for a second, the brain is tiny and surrounded by heavy bone. Brain shot is out. The leg shot is only going to wound the dino, as it can shut of bloodflow and re-route the nervous system around this injury, so the leg shot is out. I think you would end up with a pissed off T-Rex with the leg shot. So that gives us a heart lung shot. Well, the heart sits high in the chest and and lungs are much lower, so there's no way to take out both with a single shot. I personally think with as much blood as a 8-10 ton dino would have a Rex would be mobile for much longer than I would want to deal with, so the heart/lung shot is out. This points us to a spine shot right behind the skull. The bones are fairly light here and a .50BMG with solid 750gr boattails should easily get the job done. The target is about 5 inches by 5 inches, and not much different than a deer's vitals. So we need a precision .50BMG bolt action. One of the Barretts sould do nicely. You could possibly step up to the 14.4 or 20MM if you feel undergunned.
In conclusion, I think you can get the job done with a .308, .375, and the big .50. A high-cap pump shotgun with slugs, buck, and maybe #2 lead would provide camp meat, and a proper side arm would be something like the Ruger Alaskan revolver in .454, and I think I would take a .22lr 10/22 just for fun. If you want to hunt a triceratops or something then the .600NE double rifle would be handy, but the .50 can get the job done just fine too.
Now where did I leave that time machine.......
t2e