Bolters have a 'pistol grip' which means they can be fired with a single hand. This takes considerable practice and skill and is not normally encouraged. Units noted as having the 'True Grit' skill in the army list, however, have learnt how to use their bolters in this manner. In game terms, this means that they may count their bolter as a bolt pistol in close combat and will therefore be allowed to roll an extra Attack dice if they have been equipped with a second pistol or close combat weapon. However, a model using their bolter in this manner may not receive the attack bonus for charging, as a bolter is too unwieldy to be fired with one hand while simultaneously hurling yourself at the enemy.
Errata — Chapter Approved: Codex: Space Wolves clarifications (v1.0): The True Grit rule does apply to all combi-weapons, but not to storm bolters.