When I wrote How do I use elemental effects in Borderlands 2? to the best of my knowledge Explosive guns couldn't critical; I've tested this with "grenade" Assault Rifles, Gyrojet Tourge Assault Rifles and rocket launchers, none of which are able to score a critical (that I ever saw). When using a Tourge explosive Shotgun today, I started getting Criticals just like normal for a shotgun.
Which explosive weapons can cause critical hits? Is there some reason some do and some seem not to?
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.
Generally*, in Borderlands 2, Torgue explosive 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 a non-Torgue 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: "Torgue" Borderlands Wiki article (emphasis mine)
*Note that the above does not apply to some weapons:
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."
All Rocket Launchers cannot score critical hits, except on a Crystalisk's legs.
Despite a long-standing belief that the 12 Pounder was the only rocket launcher capable of scoring critical hits on a Crystalisk's legs, all rocket launchers are capable of doing so, and none of them are capable of such on other enemies.
Source: "12 Pounder" Borderlands Wiki article
Some notable explosive Unique/Seraph/Legendary weapons that can score critical hits: 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).
Every explosive pistol i have used has been able to land criticals. Same for shotguns. Never yet found a rocket launcher or explosive assault rifle that could land a critical.
