Alistair Wilson was shot dead almost six months ago

Mr Wilson was murdered on his own doorstep in Nairn last November.

The semi-automatic Schmeisser pistol murder weapon was produced in Germany between 1922 and 1930.

Northern Constabulary are carrying out further checks on the ammunition used and on internet sites connected to selling firearms.

Next Sunday will mark six months since the 30-year-old father-of-two was gunned down.

We are carrying out further checks on the ammunition at the moment Det Chief Insp Peter MacPhee

Despite a number of high-profile appeals, including a feature on the BBC's Crimewatch programme, officers have yet to catch the killer or establish a motive for the shooting.

The force has enlisted the help of a German ballistics expert in a bid to more accurately date the .25 calibre handgun.

Det Chief Insp Peter MacPhee, who is leading the investigation, said: "We are carrying out further checks on the ammunition at the moment.

"These checks are being made to ascertain the chemical constituents of the ammunition which in turn may allow us to date it."

Police are searching sites which sell handguns

The DCI also said that the team is continuing its inquiries with the makers of the ammunition, 180-year-old company Sellier and Bellot, based in Vlasim in the Czech Republic.

Internet websites featuring the sale of firearms have also played a part in the investigation.

DCI MacPhee added: "We have been looking at several websites as part of our inquiries.

"Our inquiry covers a broad spectrum, but I wouldn't want to be any more specific than that."

DCI MacPhee appealed for information on the weapon and gun on Crimewatch UK in April.

Since the programme police have received more than 40 calls, including two just last week.

DCI MacPhee said information gained from the calls, some of it going back more than 20 years, is still being followed up.