I saw J.Kb.’s post and I thought it was a good reminder why we keep shotguns without a round in the chamber.
Now let’s discuss how the safeties work on your shotgun. The two most popular pump-action shotguns on the market, the Remington 870 and the Mossberg 500. Both have safeties that engage the trigger of the gun, rather than the action.
I know I am going to use the wrong terminology, but basically the spring-loaded firing pin of the shotguns are not secured with the safety and certain movements can make it displace enough to strike the primer of a loaded shell. Do read the article in the link above for a much better description and explanation.