Farmer Tony Martin, who was jailed for shooting dead a teenage burglar, will not face any charges after being investigated on suspicion of possessing an illegal firearm.
The 71-year-old was arrested last month after a police search of his home, apparently following comments he made to a newspaper reporter about the prevalence of unlicensed firearms in the UK.
Norfolk Police confirmed a firearm seized by officers at the time of Mr Martin's arrest was "was deemed not to be a viable weapon due to its condition" and said he will face no further action.
Mr Martin served three years in prison for shooting dead Fred Barras, 16, at his Norfolk home in 1999 in a case that sparked national debate on the defence of private property.
He was initially found guilty of murder but this was reduced to manslaughter on appeal.