While there are quite a few great examples, It has been my experience since the early 80's that you will always see some Beretta O/U in one form or another almost anywhere I have been. I have never been in the Pacific Northwest, but midwest, east coast, midwest, south, and southwest Berettas are common. I have never shot in Florida, but I bought one of my shotguns from there and the Silver Dollar Firearms site seems to move a lot of units, new and used. I know that is only a 30 year slice of the ATA history pie, but I think it is significant.
The reports of the guns chosen by the competitors 1895-1902 are above; British prior to about 1896, then L.C. Smith and Parker until the repeaters (esp. the Winchester 1897) and SBTs took over. SD&G were importing Daly SBTs and VL&D Francotte SBTs by the late 1890s. Lefever was the first U.S. SBT maker (not counting Prechtel) in 1905.
Unfortunately, Sporting Life stopped rountinely listing the guns, shells, and powder used in shoots after about 1900.
Devault Infinity Ultra No need to mention any other single barrel shotgun unless you shoot doubles then I would say an MX-10 o/u , but you must add a Devault stock to the o/u or it will not be complete. Shoot one lately I do all the time I send lead to a target.
Since about 1970.. the Perazzi has lived in the winners circle.. I think the Beretta 680 series is close on its heels.. under that would be the Krieghoff.. followed by the Browning o/U.. To see #5 and 6.. It would be a sub model of the above mentioned..