Models in base-to-base contact may not fire weapons in the shooting phase. Their attentions are completely engaged by the swirling mêlée. Likewise, while especially twisted and soulless commanders may wish their warriors to fire indiscriminately into close combats in the hopes of hitting the enemy, this is not permitted. The events in a close combat move too quickly and the warriors themselves will be understandably hesitant about firing on their comrades (they may end up in the same straits soon enough after all), so just forget it, alright!
However, an exception to this rule is during a Sweeping Advance. In this case the advancing unit will be exposed to enemy fire as it moves forward – and will make a very tempting target for the unit on the receiving end of their advance. Therefore units making a Sweeping Advance can be fired at before the next assault phase is fought, even if they have moved into base-to-base contact with another unit. Range is measured to where the advancing unit has reached and even models they have moved into base-to-base contact with may fire.