Just when ya thought this thread was done
In "Arms and Armor Annual" (ed. Robert Held, 1973), there is an article titled " Body Armor in the American Civil War" by Harold L. Peterson (p.304-307).
In this the types of armor produced for the Union army were of two basic types, both produced in New Haven, Conn. about 1862.
The most popular was the 'soldiers bullet proof vest' by the G.D.Cook & Co. which was a varying size fabric vest with pockets for insertion of spring steel plates on either side of chest.
The other was by the Atwater Armor Co. of New Haven, and was more complex, actually of a cuirass form much like those of 17th-19th c. .
Obviously the biggest problem with these was weight, and there were also many homemade styles using these concepts. It is noted that hundreds of instances are recorded of soldiers lives being spared by these notably uncomfortable and awkward items, however it appears that those using them were often ridiculed as walking iron stoves etc. Many of these were discarded, but it does seem there are numbers of them in various Confederate museums, taken in battle and of course suggesting obviously that they werent always entirely successful.
Returning to the original theme of the thread, since these vests were well known nearly two decades prior to the period of gunfighters most often discussed in the 1880's to the turn of the century, there would seem to have been potential for being considered.
Best regards,
Jim