Did you happen to notice the scene in "The Public Enemy" that's very similar to the Tuco-gun shop owner sequence in GBU?
You know I'm not sure now, I could be mistaken, he's in a shop he asks to see a revolver, he asks for bullets, he loads the gun and robs the clerk, it could be from another Cagney gangster film "The Roaring Twenties".
Now, in addition to the gun shop sequence that was borrowed for GBU, I think there are two more scenes in TPE (not mentioned by Frayling) that Leone may have borrowed from in his cinema:A) When Tom is dancing at the club with Jean Harlow, he leaves with Matt to go on a job. Harlow is distressed that he is leaving her; he basically has to choose between the two, but he ditches her to go with Tom on the job. Similar to two very important Noodles-Deborah-Max scenes in OUATIA.