Depends how they did it. Shoot someone with an unregistered or stolen rifle from long distance, capture your brass, destroy the rifle and brass. Obviously a murder, if it were done that way. Very little effort involved, and what've the cops really got then?
You'd probably create more evidence trying to make it look like an accident than you'd clear up just by doing it some easier way.
It really depends on where you are. There are vast rural areas in USA, for instance, in which there are many rifles the theft of which wouldn't even be noticed for weeks. Just don't take them during the month before deer season.
