Because Metal Slug 6 is a fan favorite, many users want to play it on weaker hardware (like a Pandora's Box handheld, a Raspberry Pi, or an older PC) that struggles with Atomiswave emulation.
Would Metal Slug 6 run at full speed in MAME? - Aussie Arcade metal slug 6 mame 0139u1 rom exclusive
Here is a useful breakdown of what this specific version is, why it matters, the hardware differences involved, and how to actually get it running. Because Metal Slug 6 is a fan favorite,