Page "Gun safety" Paragraph 44

Shooting ranges may require that firearms not in use have the bolt, slide or ( in case of revolvers ) cylinder locked open to expose the firing chamber as empty.

In addition, an open bolt indicator, such as the yellow safety flag distributed by CMP may be inserted in the barrel ( needed if the firearm design lacks a mechanical hold-open device ).

This is particularly important when shooters go down range to set up, score or remove targets.