The Mac-10 gun found hidden in Swaby’s home was used in the murder of a music producer three years ago when a row over a “disrespectful” rap lyric broke out, Sutton police have revealed.

Music producer Richard Holmes, 21, was shot dead with the submachine gun by associates of rapper Dwayne Mahorn, known as MC Durrty Goodz, in Chingford, east London, on November 4, 2005.

The song, by Mr Holmes's friend, Sabar Shah, 16, accused Mr Mahorn of having “lost touch with his roots”, said Richard Horwell, QC, prosecuting at the Old Bailey in November 2006.

Mr Shah’s lyrics said: “Over the years, things change in the hood. I used to have a lot of respect for Durrty Goodz – Not no more.”

Mr Horwell said Mr Shah’s point was he felt Mr Mahorn had turned his back on the estate since his music career had taken off.

Shah refused to apologise and later that evening Mr Holmes and some friends were surrounded by associates of Mr Mahorn armed with a Mac-10.

Mr Holmes was shot in the leg and the back as he ran off with his friends under a hail of bullets.

The Common Serjeant of London Judge Brian Barker said: “This was an operation of staggering ruthlessness and arrogance.

“This was nothing but a cold-blooded killing.”

Killers Carl Dobson, 23, of Stratford, London – also known as the grime rap star Crazy Titch – and Anthony Green, 35, of Loughton, Essex, were given 30-year minimum terms at the Old Bailey.

Mahorn was cleared of both murder and manslaughter.