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Metal Slug runs in a 4:3 aspect ratio (square-ish). Modern monitors are 16:9 (rectangular). metal slug collection 1234567x pc comple link
MAME is the gold standard for reproducing the exact arcade behavior of Metal Slug 1 through 5 (Neo Geo ROMs) and Metal Slug 6 (Atomiswave ROMs). Running these games via MAME allows you to experience the exact frames, hitboxes, and audio synthesis heard in 1990s arcades. Here's a comprehensive piece on the topic: Metal