Another reason is CMP claimed this was not a USMC XM-3.
Yep, it just goes to show how little the people at the CMP actually know. Remember, they aren't collectors and historians like we are, they're just regular people working a regular job. It's understandable that they wouldn't know the specifics about certain rifles (even M83 sniper rifles are just Remington 700's in their eyes), but to sell dozens of XM3's and not know what this rifle was? That's just incompetence.
Everyone here has an advantage over the CMP and other collectors. Trust your instincts and your understanding of specific features. No one here should need someone to tell them what a rifle is or isn't. Know that the CMP will make mistakes, and that they don't have the same information that we do. They also don't care if their auction listings are correct or not, the item will still sell and there's plenty more items to sell after it.
Become an expert, be able to analyze characteristics and come to your own conclusions. This is an amazingly knowledgeable community and the absolute best advice I can give is to trust your knowledge base. Everyone here should be able to identify a real USMC XM3 and other sniper rifles within a few seconds of looking at specific features. Having a serial number list is helpful for verification, but it's not necessary to have in order to identify these rifles. If it looks like a duck, walks like a duck, quacks like a duck, it's a duck.
There are things with some CMP auctions that are on their site right now that are hidden to everyone except the most astute among us. Not just entire rifles being mislabeled, but very small details that no one would really think about. See if you can find these things in the current CMP auction listings. Analyze specific details and understand what's going on with certain items. Everyone can test their skills and post their findings, if they choose to do so. That's all the clues I'll give for now.
@USMCSGT0331 can you post all the known XM3 serials? Would be a good thing to have for reference as you take a step away. Much appreciated!
It takes a lot to get very specific information that no one else has, so it can be difficult to just give it away. For some information, myself and others have been asked to keep information private. It's just something that we had to agree to in order to have access to the information. The reasons for this request can differ from person to person, but the outcome is the same. Most of the time it's not that we don't want to share information, it's because we can't share information. Breaking this trust isn't just dishonorable, it's also an instant blacklist.
I'd love to share this serial number list, and other serial number lists, but at this time I can't. However, in time all of this information and more will be released.