I cannot find any information about a "12 7 X 42 caliber." There is no standard ammunition or firearm using this designation. It's possible:
- There's a typo: The caliber might be slightly different (e.g., 12.7x42mm, .12x70, etc.). Please double-check the source of this information. The inclusion of "X" is unusual in caliber designations.
- It's a custom or obscure round: Some custom ammunition manufacturers produce rounds not widely documented. Finding information about these would require contacting specialized firearms forums or manufacturers.
- It's a fictional caliber: The designation could be from a book, movie, or video game.
To help me find the information you're looking for, please provide:
- The source where you encountered this caliber: Knowing where you saw this term will help determine if it's a typo or a legitimate (though obscure) designation.
- Any additional context: Is this related to a specific firearm, manufacturer, or application?
With more information, I can try to provide a more accurate and helpful response.