That, and it's hard for Ripley to get a clean shot since she's not an experienced marksman and the Alien moves very fast. Though an In-Universe explanation might be that the revolvers are very weak and weren't really meant to shoot anything that was built to last. There's only one Alien in the game Most of it, anyway, and if you could only take it out with just a few bullets, then the game wouldn't really have much else to go on.
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