One other thing to factor in is the development of low maintenance firearms. That required the development of smokeless powder in the end of the 19th Century a

One other thing to factor in is the development of low maintenance firearms. That required the development of smokeless powder in the end of the 19th Century and "non-corrosive" primers in the 20th Century, although per Wikipedia it appears that mercury fulminate is non-corrosive but not as shelf/environmentally stable as the corrosive compounds. Non-corrosive primers were state of the art in 1911 when the Swiss adopted them in their service ammo, but widespread adoption in the US was probably post-WWII. (The Soviets addressed this by among other measures chroming the important surfaces of the AK-47.) So nowadays, unless you're using old surplus ammo, likely for a rifle, a gun is going to stay functional for many shots with little or no maintenance (like 100s). Thereby providing a lower threshold for the discipline required to employ one.