Firearms are difficult, or impossible, to obtain. Criminals have used reactivated guns, modified replicas and modified starting pistols. Therefore, any proper firearm is like gold dust and a hitman, or equally just someone who wants to carry out a murder, would gladly use such a gun, if available. It also has the advantage of being relatively quiet.
Definitely agree with this regarding an ‘any gun will do’ approach, but the use of such a convoluted strategy seems perverse to me. Shootings in Scotland tend to be quite straightforward affairs, these guys typically aren’t hanging around on doorsteps with cryptic envelopes - I can’t think of another killing like this one, in all honesty.