Let me preface what I am saying here with the statement that I have zero experience with original guns with barrel keys and that I know what I have done on a few rifles now is not historically correct. In my experience I have not been talented enough to get the keys to consistently fit perfectly to begin with ....and when I have (mostly by accident) the fit eventually loosened by virtue of wear or humidity changes, etc., etc.  And it seemed to me that even if I captured the key, the fit (too loose or too tight) would still be an issue.  So I did this....take it for what it's worth.
From an old post of mine..........
Thanks to all of you for the help and advice on fitting up barrel keys.  I am trying all of them, but, in addition, I decided to modify the keys to include a fairly stout friction spring.  I cut a shallow slot in each key, made a leaf spring for each one and then riveted the spring in place using a brass tack as a rivet.  The keys slide in fairly easily but hold very firmly.  Not HC, I expect, but I like things that are self adjusting and won't be as subject to humidity and temperature changes as I have experienced thus far with these keys. 

