In every ROM prior to the 0.139u1 update, the game would frequently soft-lock during the branching path in Mission 4 (The Iceberg stage). The 0.139u1 driver specifically corrected the DMA (Direct Memory Access) timing for the Atomiswave cartridges, eliminating that crash entirely.
. If you are looking for a "better" experience with this specific version, you are likely encountering either the Atomiswave-to-Naomi conversion or a specific bootleg ROM hack The "Better" Version for MAME 0.139u1
The authentic version of the game. However, in older MAME versions like 0.139u1, this version is often listed as "not working" or runs at extremely low frame rates (10–15 FPS) because Atomiswave emulation requires significant processing power.
Optimizing Metal Slug 6 for MAME 0.139u1: The Ultimate Guide