Why do they call the PPsh41 the Burp gun?

Because the blowback made it sound like a belch when it was fired.

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Agree. The Sphagin was ONE of the guns called a "burp gun". The slang term originated in the period 1935-1940, when referring to a high rate of fire machine pistol. The 800 rounds per minute of the Russian machine pistol sounded more like an extended BURP than a BANG. So- burp gun.