An appeal to find 40 guns believed to have been converted into lethal weapons in Berkshire has yielded few results.
Police offered a £10,000 reward for any information leading to the recovery of the replica Mac-10 sub-machine guns.
The weapons were converted by Grant Wilkinson in Three Mile Cross, near Reading, with some used in murders. He was jailed for life last month.
But so far none of the guns have been found, although a machine gun recovered on Thursday is being tested.
The Metropolitan Police said forensic scientists were looking at the weapon to determine if it is one of the 40 outstanding.
Eight murders
Det Supt Gary Richardson says: "A reward of up to £10,000 is being offered to anyone who can provide information which leads to the recovery of one or more of these weapons or leading to the arrest and conviction of any individual concerned in these historical offences.
"We are researching and reviewing all of these offences to identify opportunities to recover those outstanding weapons and to target those concerned in offences linked to these weapons."
Wilkinson, 34, was found guilty last month of turning replica guns into weapons linked to more than 50 shootings, including at least eight murders.
He was convicted of seven offences of converting and supplying firearms, including one linked to the murder of Pc Sharon Beshenivsky.
Wilkinson, of no fixed abode, adapted replica Mac-10 sub-machine guns in a shed in Three Mile Cross.
He was sentenced to a minimum term of 11 years at Reading Crown Court.
Co-defendant Gary Lewis, 38, of Bourne End, Buckinghamshire, was cleared of all charges.
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