Here is the prepared content regarding Metal Slug 6 on MAME 0.139u1.
If you experience low frame rates (common with Metal Slug 6 on older MAME versions), many users recommend: RetroArch Flycast Core
Platform Alternatives: Since the Atomiswave is very similar to the Sega Dreamcast, you can find community ports that run perfectly on Dreamcast emulators.
The proper ROM set for MAME 0.139u1 had to be decrypted and reorganized to match MAME’s expectations. Early dumps were bad or incomplete. The correct set had a specific CRC32 and SHA1 hash that matched MAME’s mslug6.xml driver.