If a guardsman is engaged in melee, may he make a ballistic skill attack at the enemy with which he is engaged? What would the modifiers be? Thanks.
In melee you can make shooting attacks. But only against the target you are in melee with and only with pistols or weapons deliberately stating the can be used one hand in combat. There is no additional modifiers for shooting your melee opponent with a pistol, but note that you do not get the bonus for Point Blank Range in melee. If you are using Two-Weapon Fighting penalties will apply (see page 252 for Two Weapon Fighting).
Firing at a target you're fighting in Melee is exactly the same as shooting at any other target except that you can only use Pistols to attack with and you ignore the +30 bonus for being at Point Blank Range. Everything else is the same, whether it's making a Standard Attack (+10), shooting a target of Enormous size (+20), making any test while Fatigued (-10), shooting at a target in utter darkness without equipment to help you see (-30), etc. etc.
The only Range modifiers you need to keep in mind are the following:
(1): if the Regiment have the Close Quarter Combat Doctrine from Hammer of the Emperor (+10 bonus to BS skill checks made at Point Blank range).
(2): if they are using a Pistol with the Scatter Quality, such as one of the Shotgun Pistols from Dark Heresy, in which case they would get the +10 bonus for firing a Scatter weapon at Short or Point Blank range and the +3 damage bonus for firing a weapon with Scatter at Point Blank range.
(3):
Doubling the Pen of Melta weapons for being fired at Short Range or closer, in the unlikely event that your players have gotten ahold of an Inferno Pistol.
Or so I thought, re-reading the Weapon Quality says "This weapon doubles its Penetration when firing at Short Range.", it doesn't mention if the bonus also apply to the Point-Blank range.
Well, I guess this is the place to ask: Does the Melta Weapon Quality give the bonus to Pen when fired at Point-blank Range?