If you have the correct bayonet for the rifle you do not need rings. Otherwise wrap the muzzle end of the barrel with masking tape. It will do no harm to either the rifle or the bayonet.
Dan
This will work, in the short term, however my recommendation would be to remove the tape as soon as possible after you are done.
High tack masking tape is a bear to get off after it has been on something for weeks/months/years.
The paper part degrades but the adhesive seems to be eternal, it has also been known to discolor some materials, unsure about blueing or phosphate finishes but who wants to try.
If this is just for display purposes then put it on, do the show and get it off.
A rubber o ring or, more likely some synthetic polymer ring, these days, might provide enough tension between the ring on the bayonet and the stop on the barrel but that also should be removed ASAP.
Moisture will collect under the ring and will discolor the blueing or, possibly, rust.
This method might also make the bayonet very difficult to latch.
I'm curious though, why is this a concern?