It’s not surprising in a blowback gun, especially with a pistol cartridge.
Think about it: The breech isn't locked. The bolt is only held closed by its own mass and the weight of the recoil spring. Which is going to give first, the recoil spring or the walls of the barrel? In this case, the bolt is acting as a pressure relief valve.
Assuming someone who didn't notice that there were no holes appearing in the target was capable of noticing anything, he'd notice more flash and ejecta from the ejection port with each shot, accompanied by the brass being flung out of the gun with increasing force on every shot.
Similarly, someone stacking bullets behind a squib in a revolver should theoretically notice constantly increasing flash from the b/c gap with each shot.
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