I cannot imagine horror of the precedent it would be set if H&K successfully sued AB over copyright infringement for names that are visible only to the player who entered them. Those names are not shown publicly.
H&K has an US trademark consisting of just "MP5" in relation to a ton of things (though not video games!) so they could at least try make a case out of it not being purely nominative use and tie AB in court, if they wished. It would be PR suicide, but still, not the most stupid thing they have done.