The only "old" gunshop I could recall was one at Milwaukee and Ogden and Chicago, about 750 niorth on Milwaukee ave. It was a block from the bowling alley where Machinegun Jack McGurn was gunned down on the second floor in 1931. The building was under the sane ownership since then, perhaps, as the first floor business operator showed me a composite news and picture montage of the 1931 incident. He said part of the bowling alley floor was still up there. Crime history photos will sometimnes show a nicely clad body in good three piece clothing on a wooden floor with a raised section of benches for spectators in back. I once saw a loose actual photo of that type which was unmarked but showed this nice young Rudolph Valentino type on the floor with the wrought iron metalwork and benches in the rear. I put two and two together to determine that was McGurn (who was actualkly italian).
I passed the Milwaukee gun shop on the streetcar (and later the busses when those started) and at the time (perhaps age 8-12) had no interest of a more scholarly type in guns. The place must have had fabulous things in terms of American history and "ol' timey" American stuff go through there.
Incidentally one of my 1977 paperback crime histories of the Capone era did mention a gun shop of some age which I believe was on State street and which may have provided Thompson machine guns to the mob. I happened to buy an Enfield jungle carbine at the west Washington St. shop downtown where Lee Harvey Oswald bought his mail order Carcano.