Qfyilyi

In the movie Jurassic World there is a scene where a specialised military response team engages the Indominus Rex, a genetically modified dinosaur said to be of comparable size to Tyrannosaurus Rex. The weapons they carry are largely typical late twentieth to early-twenty-first century firearms, and even the crew-served weapons - including some kind of machinegun fired from a helicopter, don't seem to do any significant damage to the dinosaur.

This has some precedent, in that even real-world large mammals have proved challenging to dispatch using firearms - the elephant gun was developed specifically for hunting the largest of game.

A realistic dinosaur of the genus mentioned previously would probably weigh somewhere in the region of 15,000kg, whereas the largest African elephant weighs around 6,300kg. This indicates that the movie drastically over-represents the size of a Tyrannosaurus-type dinosaur, which appear in the movie to be the height of a three or four-storey building at least and massing at a rough guess at least ten times their real-world size.

For the sake of the question, let us assume that we are aiming to deal with a movie-type dinosaur - a very large and very durable animal, well in excess of the largest terrestrial species to have been killed with a firearm.

How powerful would a firearm have to be to kill this hypothetical dinosaur with one to three shots placed with skill, and what technological adaptations would be required to make this weapon readily portable in the manner of some kind of small arm? By small arm, I mean a weapon such as a rifle or shotgun, or exotic variations such as gyrojets, with automatic or semi-automatic loading from a magazine, potentially specifically-designed to kill an extremely large animal if this is necessary to achieve the goal. The weapon should be practical to carry and fire on the move by a single operator, including while under attack by the animal. Is such a thing feasible?

Go hunting with the appropriate weapon type and tactics

Dinosaurs are equivalent to lightly armoured vehicles (albeit very large ones) in terms of durability. So the best tactic is to use the same weapons that have been developed for that purpose today with the associated tactics.

Tactics are the first consideration. When hunting armour, an outright kill with the first shot is desirable, but immobilising the target (a "mobility kill") is almost as valuable. With dinosaurs that have no ability to shoot back, a mobility kill is even more desirable. Mobility kills are also much easier to achieve for those lacking detailed knowledge of dinosaur physiology - faced with a dinosaur I would have no idea where the heart(s?) or brain (s?) are located, but the spine and joints are pretty obvious and could be damaged by light anti-armour weapons.

Regarding the weapon choices - my suggestion would be to go with 40mm grenades as an easily man-portable option with a relatively high effective rate of fire. The M430A1 HEDP round can penetrate 76mm of steel plate, which should be ample to penetrate the skin and shatter bones for most dinosaurs. The grenades can either be launched from under-barrel grenade launchers (if the troops also need standard rifles to deal with small dinosaurs) or from the Milkor MGL, which is a six-shooter. One or two shots to a leg to immobilise the target, then a carefully aimed shot to the spine and the target will die.

For those who want something heavier with a longer range, go with the Carl Gustav 84mm. The FFV551 can penetrate up to 400 mm of RHA, so even the toughest dinosaur will have a large hole blown through it, wherever is hit. It can target stationary targets out to 700m and moving targets to 400m (some ammunition types can go out to 1000m), so this is the weapon of choice if long sight lines are available. However, the weapon and ammunition are much heavier and the back blast danger area is significant, so this is ideally used by sniper teams in fixed positions rather than for patrolling.

The answer was already given in Jurassic Park

The Lindstradt Gun loaded with cone snail venom.

Whilst this particular gun is fictional, the snail venom is very real. A dart gun could easily be loaded with this venom.

Standard 5.56 or 7.62 caliber rifles would of course not be effective against a large dinosaur, unless the eyes are successfully targeted.

5.56 or 7.62 caliber machine guns, while not immediately lethal, would be quite annoying, and after a big enough number of hits, our dinosaur should bleed to death.

0.5 BMG (12.7mm) machine gun should have enough power to penetrate internal organs, and with some luck, drop the dinosaur within several seconds of firing. But this caliber is still insufficient for a quick and reliable kill.

0.5 BMG sniper rifle probably can kill the dinosaur in a few shots, but this is again won't be a reliable kill.

Shoulder-fired anti tank missile have the greatest chances of killing a large dinosaur in one shot. The wounds would be deep and extensive, and even if vital organs are missed, one wound would likely be incapacitating. However, due to cauterizing, bleeding would be relatively low.

As others have said. You'd choose weapons that were effective. Very high energy 'dumdum' style rounds would cause quick incapacitating damage. Aim for the legs. If you could guarantee a headshot.. a single 50 cal dumdum sniper round would pretty much remove the head of any dinosaur. If you were being chased.. had one shot and absolutely must stop the beast.. an RPG with a shaped charge aimed at the torso would work. Or a minigun and a lot of swearing.

Very Feasible

According to this article, you could quite likely do so with a standard assault rifle (7.62mm), with good placement, as the skull of the T-Rex at least, and probably a good many other dinos, is full of holes, and so shots could be fired into the creature's brain without having to contend with bone.

Alternatively, something beefier, such as a .50 cal or 20mm (or smaller, high grain count ammunition) could rip through flesh and bone for days, very likely passing through a dinosaur skull with little effort, assuming your aim was less than perfect (and with the recoil of such a round, that's a reasonable assumption).

Point being, firearm technology has advanced considerably since the introduction of the elephant gun, and with the use of specialty rounds, such as armor piercing or so called "cop killer" (or Black Talon) rounds, you could probably do it with a hand gun if your aim was true (through the eyes, nostril cavity, soft palette, or carotid artery). Quite probably, with a 10-12 round clip of armor piercing rounds, you could probably reasonably rely on enough internal organ damage to kill it with only body shots, though this might be too slow to prevent it from taking you with it...