Venom Cannon
From Exterminatus Manual
Venom Cannons are the long range weapons of the Tyranid Carnifex , firing electrically charged crstyals of toxins.
The venom cannon allows a carnifex to engauge in long range combat, keeping him out of range of the specialists effective counter attacks. The crystals shattter on impact, scattering razor sharp poinsionous shareds over a large area, making it effective agains clustered of targets. However a direct hit from venom cannon can carry enough power to kill even an armoured targe in a single hit. The Vennom cannon is heavy and cycles quite slowly, sot it is worth taking your time to aim.
Tactics
Keep back from the fighting and sap the enemy with the venom cannon, which has greater range than the imperial boltguns (but not heavy bolters, missile launchers or plasmaguns), this will draw the enemy in so close range tyranid classes can ambush them.