While shooting on a friend's backyard range this past weekend, we were using in-bore chamber flags to mark firearms whose bolts couldn't be locked open to show clear. It reminded me of this very cool product purpose-built for this exact thing.
Chamber-View makes a series of bright orange chamber inserts that lock bolts open with a physical barrier demonstrating cleared actions. Unlike traditional chamber flags that insert into the chamber and allow partial closure, the Chamber-View inserts also physically block the action. This prevents dirt or dust from getting into the action while the firearm is cleared.
Inserts are available for both pump-action and semi-automatic shotguns, AR-15-platform rifles, rimfire firearms and handguns chambered in 9 mm and .40 S&W. Chamber-View will be expanding their product line to include bolt-action rifles, 1911s and numerous other action types.