Weapons can have a very long life span, just look at the M16. Its fully possible any of those guns existed in 2550. Regardless, doesn't the wiki state when it entered service? I know it states when covenant equipment appeared through their naming conventions, isn't there something similar for UNSC weapons? Or have we never been told that?

Also, Halo Reach era gear would be the same sort as CE, two and three. They all take place within quick succession of one another, barely spanning a few months in total. I find it difficult to believe that any gear we were using in Reach would have been phased out so quickly. And there's only two years from this RP to those games.