What exactly is the concern? Is the concern with the rifle as a firearm, the bayonet as a sharp blade, or both?
With the bayonet it will be similar to swordplay on stage. Assuming there will be an experienced fight choreographer to stage the action, scheduled fight calls ahead of performances, and secure storage and proper chain of custody before/during/after performances, the risk should be mitigated to a level consistent with industry standards.
With the firearm - these are WW1 rifles. Is the concern that the specific actors handling the firearms will bring in rifle cartridges of the exact size and calibre needed to fire them, then load and fire the rifle with live rounds?
Most, though not all, WW1 rifles used ammunition of a calibre not used in modern rifles. Some WW1 rifles did use ammunition that is common today (.30-06, for example), but I would argue that anyone who is familiar enough with firearms to figure that out also knows that firing a 100+ year old rifle without inspection from a qualified gunsmith is a BAD idea. You risk the rifle quite literally blowing up in your face.
Playing the firearms thing out to its logical conclusion, I'd argue you're far more likely (though still not at all likely) to have an actor simply bring a modern weapon into the workplace and use that.
As Matthew has said there is no way to eliminate risk 100%. The job is to mitigate risk to an acceptable level.
Some thoughts on specifics:
- Insist on provisions for a qualified fight choreographer (if that person doesn't already exist).
- Schedule adequate staging time. Schedule fight calls before each run and performance.
- Have secure storage for any weapons while they are not in use. This would be a lockable area that only necessary people have keys to. Each key should be numbered and inventoried, with documentation kept to who holds which key. If it's a long run, you should schedule periodic audits of keyholders and records.
- Have a clear chain of custody any time the weapons are not in storage. They shouldn't be left on the props table, for example. This should be documented and enforced. Similar to above, periodic audits for long runs should be conducted and documented.
- If the firearms will be fired using blanks, the blank cartridges should be inventoried. Prior to the firearms being loaded, two people should verify the number of cartridges loaded, and that all cartridges have come from the secure storage area. When the firearms are being loaded, a second person should be present and verify that only inspected blank cartridges have been used.
- If the actors in question have not used firearms before, it would be a good idea to have a familiarization with firearms in general. Find an instructor and have the actors fire some rifles (not your WW1 props!) at a target range. [Sane and rational] People who use firearms develop an immense respect for the power they have and the damage that can be done by careless use.
- With the bayonets, if they are sharp - have a day where they actually use the bayonets (think watermelon or pumpkin). Again, when they see it cut through a pumpkin they will understand what it can do if they are careless.
Good luck - let us know how it goes.