I think with such a system if you forgot to lock it when the powder fired the loose bolt would move back allowing the gas to quickly expell in a flash....... someone might get a powder burn but I doubt that there would be a catastrophic failure.......
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If you really have to have a safety then build it into the bolt head where it pushes agains a cam allowing the hammer to strike, if the bolt is not fully closed no matter what you do the hammer won't fall.....