Experts at the National Ballistics Intelligence Service (NABIS) in Birmingham have examined ballistic material from two murders in the West Midlands in the last few years which were carried out using obsolete firearms.
They also suspect a third killing was carried out using an antique gun.
Under the 1968 Firearms Act obsolete caliber firearms can be purchased lawfully without a licence if kept as an ornament or curio.
It only becomes a criminal offense if the person with the weapon also possesses compatible ammunition.
Nine Gun Calibers Under The Spotlight
The West Midlands Police and Crime Commissioner, David Jamieson, has called for the antique gun exemption to be removed for nine gun caliber groups.
Caliber refers to the size of the ammunition needed by a gun and calibers for handguns range from the smallest — 2mm — to the largest —.500, which is used in the world's most powerful production revolver, the Smith & Wesson Model 500.
The most popular caliber is the 9mm and the.45 but some calibers have gone out of production and become obsolete.
One of the obsolete calibers is the.44 Russian but in 2016 Carl Campbell, 33, was shot dead while sitting in a car in West Bromwich, near Birmingham by a gunman using a Smith and Wesson.44 Russian revolver.
Campbell's killers were jailed for life last year.
Derek Myers, 25, was also killed with an antique firearm in Birmingham in 2015.
Ten Antique Guns Fired In West Midlands
Between April 2017 and March 2018 it is thought 10 antique firearms were fired in the West Midlands.
The loophole in the law was highlighted in a BBC Panorama documentary on Monday, August 20.
A trial at the Old Bailey in 2007 also showed up this "gray area" in the Firearms Act.
Mick Shepherd was arrested at his home in Dartford, Kent, in September 2006 amid a blaze of publicity but was eventually cleared of all 13 firearms charges.
The trial heard the 900 guns he possessed were all antiques and he was perfectly entitled, under Section 58 of the Act, to own them.
WATCH NOW: Panorama investigates how antique guns are being brought into the UK perfectly legally and ending up in the hands of criminals.
'Deadly Weapons'
"It is vital the government make it much harder for gang members and criminals in the West Midlands and beyond to get their hands on antique guns which can be turned into deadly weapons with relative ease," said Mr. Jamieson.
NABIS has submitted evidence about certain obsolete calibres to the Home Office and West Midlands Police hopes they will consider removing them from the obsolete caliber list on public safety grounds.
Mr. Jamieson said people should only be able to possess certain obsolete caliber weapons if they possess a firearms license.
"The government needs to ensure obsolete caliber firearms don't fall into the wrong hands…We must not lose any more time. The law needs changing. Lives literally depend on it," said Mr. Jamieson.
But Diggory Hadoke, a dealer in antique sporting guns, said he did not think antique handguns were a big problem.
"If I was a criminal and I wanted a gun there are far easier ways of doing it than finding an antique weapon and then somehow getting the ammunition to fit it. It's far easier to burgle a house, steal a shotgun, saw it off and use that. It would be more reliable and more effective at close quarters," Mr. Hadoke told Sputnik.
"It is true that you can buy a.44 Russian caliber pistol but to do anything else is to commit a crime and adding another layer of restrictions on those people who are not involved in crime is not proportionate. It really is very difficult to make the ammunition to fit these guns," Mr. Hadoke told Sputnik.
He pointed out that just because a gun was a certain caliber did not mean it would fit into any gun.
"If you look at a.65 Mauser and.65 Mannlicher they are completely different shapes and the same bullet will not fit into both," he told Sputnik.
UK Ban on Handguns Since Dunblane
All handguns have been banned in the UK since 1996, in the wake of the Dunblane massacre, in which 16 young children and a teacher were shot dead in a Scottish school by Thomas Hamilton, who committed suicide.
Future tennis star Andy Murray, who was eight at the time, was in the school at the time of the shooting.
Antique weapons include flintlocks, muzzle-loading guns and anything of obsolete caliber.