Which explosive guns can cause critical hits?

Explosive shotguns fire pellets like all the other shotguns. Upon hitting the target, the pellet applies the explosive element at the point of contact between pellet and enemy. This means that if the pellet hit a critical part, it cause the explosive damage to be critical.

Grenade assault rifles and rocket launchers won't get critical hits bacause it's firing a projectile that explodes upon contact with the enemy. Even if you aim for the weak spot on a huge enemy (crystalisks), it won't apply the critical hit.

In Borderlands 2, Torgue weapons have two different firing traits:

• If the weapon spawns with a barrel component of Torgue manufacture, it will shoot folding fin rockets. These rockets cause very high damage, but have high spread and low velocity. They also cannot score critical hits and each rocket costs multiple units of ammunition. In this case, the explosive damage is included in the damage stated in the weapon card, and calculated by the game as a single hit.

• The other firing trait is present when the barrel component is from any other manufacturer. In this case, Torgue weapons will shoot Gyrojet projectiles. These projectiles have low initial speed, but accelerate as they travel forwards, and are faster than the folding fin rockets. They also deal additional explosive area-of-effect damage on impact and can score critical hits. The game calculates the bullet impact and the explosive effect separately (card only shows projectile damage), and the blast can hit enemies near to the point of impact.

^{Source: Borderlands wiki, "Torgue" article (emphasis mine)}
Note that the above does not apply to some Unique, Seraph or Legendary Weapons.

Assault Rifles

The non-Torgue manufactured, Assault Rifles titled: Cannon (Manufacturer: Jakobs, Barrel: Torgue) and Grenadier (Manufacturer: Dahl, Barrel: Torgue) shoot grenades, instead of folding fin rockets or Gyrojet projectiles. They act like folding fin rockets, in that they also cannot score critical hits.

The Seraph Assault Rifle, Seeker, despite being manufactured by Torgue, and having the barrel not made by Torgue, cannot score critical hits. Weapon special effect description: "Oh, yeah, that's fair... – Shots fired home in on enemies. Each shot consumes 2 ammo."

Rocket Launchers

Rocket Launchers cannot score critical hits. An exception is the 12 Pounder, a Unique Rocket Launcher. It is the only Rocket Launcher that can score critical hits. Weapon special effect description: "Nec Pluribus Impar, Bitches. – Increased damage, reduced magazine size, and fast reload speed. Fires arcing cannonballs that explode if they directly hit an enemy or bounce with parabolic trajectory if they hit an inert surface before exploding. Cannonballs are capable of scoring Critical hits".

Unique/Seraph/Legendary weapons

Some notable explosive Unique/Seraph/Legendary weapons that can score critical hits: 12 Pounder (Manufacturer and Barrel: Torgue), Cobra (Manufacturer: Jakobs; Barrel: Dahl), Ogre (Manufacturer: Torgue; Barrel: Vladof), Unkempt Harold (Manufacturer and Barrel: Torgue).

Some notable explosive Unique/Seraph/Legendary weapons that cannot score critical hits: KerBlaster (Manufacturer and Barrel: Torgue), Seeker (Manufacturer: Torgue; Barrel: Vladof).