Combat in w40k is identical to that listed in T20 with the following additions.

A lot of weapons in the Imperium are hold-overs from the time when man ruled the cosmos as a high tech Empire (imagine the Traveller Imperium but at TL 16+).  That means a lot of weapons are just maintened and are almost irreplacable.  After centuries (or longer) of wear and tear they have a chance of not working. 

If during combat the player rolls a critical failure while using a non-slug throwing weapon (this excludes the high tech Gauss rifle) the following happens:

Gauss based weapon: The weapon jams.  It takes a move equivalent action to unjam the weapon.

Plasma weapons: These weapons get hot! If the player rolls a 1 he must make a reflex save vs the damage roll of the weapon or drop the weapon and suffer half that damage.  If the character saves he only drops the weapon.

* NOTE: If the player ever rolls a 1 twice in a row to hit the weapon is breaks.  This applies to ALL weapons, not just high-tech ones..