8 ga, with 2 oz. slugs ,the heaviest slug they make in 10 ga. or if were further into the future. borrow what the terminator asked for at alamo gunshop. "a phase pulse plasma rifle in the forty watt range" . that should kill and cook at that same time. just walk up after 10 minutes and pull breakfast,lunch and dinner right off the bone.
I would take a .50 BMG, shoot it in the eye or something.
Funny story, I had a dream the other night. My bed is right next to the window, and I dreamed a Rex was trying to get me. I shot all my 00 buck at him, which failed, then I hit him once with my Glock 23 (.40 S&W) and he dropped like a rock.
T. rex, eh? Trouble with a T. rex is that his major nerve plexus is located in his lower spine. Just like a snake you can cut off his head and he'll still attack you. Heart/lung shots are useless as just like a snake he can have his heart cut out and he'll still attack for 6-8 hours.
So I was thinking that you'd need at least a 20 mm truck mounted Vulcan loaded with alternating HE and AP and maybe a tracer every 20th round. They're hard to come by so you might have to settle for an old 20mm Oerlikon. Funny nobody ever seems to put those on their Christmas wish lists any more so you might get a good deal on ebay. The $220 tax would be pretty cheap compared with the ammo.
Cheytac .408 from a hot air ballon followed up by a A/MH-6 Littlebird minigun flyby followed by a M270 MLRS 3 missile barrage to add that smokey flavor
No gun for me. I'd fill a backpack full of poison, strap it on the nearest mall-ninja and tell him to go shoot it with his brand new extreme tactical whatever.
I've always thought the old Brits had the right idea with their 4-bore doubles. Should be good for any dino you're likely to come across. That is, if it doesn't break your shoulder. LOL!!