I'm starting DH again and i'm trying to catch up with all the rules changes. I have read this thread https://community.fantasyflightgames.com/topic/41173-official-rules-clarification-about-using-pistols-in-melee/ in which a game designer says the pistol can make only single shot in melee combat. Is it official? Is there any errata downloadable about it? And if you have tried to introduce it in you game what kind of improvements you have noticed? Thank you in advance

It is official as any Dev-made FAQ, but note that the later gamerules go the other way. In Black Crusade, Only War and in DH 2.0 you are free to use pistol-size weapons in melee with no restrictions on fire-modes. Range modifiers do not apply, and some upgrades have no effect IIRC.

However, these games also have changed rules for Melee attacks (where Swift and Lightning attacks work just as Semi and Full Auto Bursts), so if you only change one element you risk upsetting the game balance.

My recommendation would be to go with the later games combat rules entirely. See my Sig for my own implementation of this.