I have a Sako AI, with a aftermarket stainless barrel, where the caliber is stated as 223K&M. Thats all thats written on the barrel.

Has anybody heard about this before?

Or, might it be a certain neck/trim dimension?

Maybe a pm to Dave Manson might turn up a project the gentleman has worked on for someone.
There is also a K&M Arms, though l see no mention of any changes they may have made to the standard 5.56x45/.223.

Does a factory 223 chamber? Might do a chamber cast and see what you get.

A very valid point, and one that l too would go with.

lf it were to be fired, what would the worst case scenario, an "Improved" or slightly altered case, which could then be checked against a factory round/case