Using personal contacts, I have spoken to several senior police officers both in England and Scotland. They all told me the same thing: the police test is purely to establish whether it is reasonable to take any further action and especially whether to spend several hundred pounds of public money in sending the gun to a Home Office accredited forensic service. There is no need in law for the police test to take place at all and guns could be sent straight off to the forensic service.

The only tests that have validity in law are those carried out by an accredited forensic/ballistic service, and part of the accreditation is calibration of the instruments used. This is one reason why police forces don't publish details of their testing procedures, what pellets they use or anything else - they don't need to, as their test is purely a police internal procedure.

The only testing procedure I was able to lay my hands on (from a large English police force, dated 2013) was clear that this particular force used a Skan Mk 10 set 1 metre from the muzzle of the gun and used a range of pellets of different weights and types 'based on experience and advice from the manufacturers, but always inculding any pellets found in association with the gun'.

Hope this helps, and please don't shoot the messenger - I am just passing on what I was told.

Alan