MP means the barrel has been Magnetic Particle inspected...part of the MilSpec process. All Colt AR/M4 barrels are done as far as I know. It's one thing that makes Colt rifles stand out from the rest...true MilSpec...many, if not most of the other brands are not. It doesn't necessarily mean other brands are not good quality...it just means the makers have not gone to th expensive process of verifying it. With the other brands that don't test for MilSpec you don't know whether you're getting a quality barrel or not, a product "seconds" or a piece of junk. You're depending on the maker's reputation and promises of quality.