The Spanish Civil War was Anarchists, Communists, and Liberals vs. a Hitler and Mussolini backed fascist Dictator, Franco.
The resistance had popular support, but the lack of weapons did severely hinder the left's effectiveness, and caused them to become reliant upon Soviet Russia for weapons supplies, which ultimately spelled their downfall as the Soviets betrayed everyone else and began rounding everyone up to execute them.
I'm not saying having a surplus of small arms in the hands of the leftists would've changed the outcome of the war (they also needed tanks, airplanes, and artillery), but it would've been a pretty big asset, had they been able to fulfill those other needs some other way as well.
Paul Harrell tested birdshot at 100 yards. It fails to penetrate a t-shirt, and gets very few hits at that range.
LuckyGunner tested Buckshot at 100 yards. It was still deadly, but few pellets will hit.
Shotgun Slugs are effective out to 100 yards.