Many thanks for the suggestions and information. Publishing isn't really on the cards; it's a method to survive the terminal boredom and excessive human interaction of working on the till for a couple of hours a day.
The story is set in December 1937, so the methods and weapons available will reflect this. There are two guns involved in the story, the Gewehr 98, with a cartridge (at least unless I find a more specialised German sniper rifle from this period) and the Japanese made Type 97 sniper rifle. The G88 used a 7.92×57mm cartridge and the Type 97 took a 6.5×50mm cartridge. Both of these are distinct from the .303/7.7mm cartridge used by the British Army at the time.
In the story the Type 97 is used to assassinate a policeman; the sniper hits on the second attempt, and the bullet that missed passes through the truck bulkhead to be found later in a damaged state when a colleague searches the truck. In a second assassination attempt on this colleague the sniper misses and hits a pile of grain sacks, allowing the shell to be retrieved in better condition. As the intended target of the assassination is a policeman he'll automatically want to identify the bullet, and those differences are fairly easy to recognise using a caliper tool.
The Gewehr 98 is fired by a German sniper, as covering fire. It is deliberately aimed at a wall next to the assassin, in order to hinder or prevent later identification of the bullet. As the sniper is firing over a distance of about 50m and the effective range of the rifle is about a kilometre, I expect the bullet will hit the wall with sufficient force to crush it. As it's a pretty mucky dockside it would be hard to find the remains in the general clutter.
