Nearly 3,000 guns were lost by or stolen from people registered to hold them in the past five years, according to figures obtained by the BBC.
"There are nearly two million licensed firearms in the UK and 3,000 represents a tiny 0.15% of that figure," said Mr Harriman, who is also a forensic firearms examiner.
Overall, gun crime in England and Wales is falling. There was a 19% drop in annual firearm offences according to the latest figures.
"The problem with shotguns is they don't leave unique rounds like firearms do. But each quarter, we probably recover about 150 shotguns notwithstanding the other firearms material."
He said guns could be purchased for a few hundred pounds in many parts of London. "You can hire a gun for a period and, if the gun has already been used for a murder, the going rate comes down."
But imported weapons, mostly emanating from eastern Europe, remain the biggest threat.