Quite correct old man. I had a batch of those supersonic ones left over that are no longer tournament legal. Only problem, when one shoots over 400 fps, is the pesky things have a habit of lifting!
I shot down a Cessna, two magpies and the Goodyear Blimp last time I used em.
Seriously, although you stated that you chrono at 300 fps, you are in a relatively confined space, and 2 velocity creep is quite common so never assume your velocity is OK (I got hit by hot Spyder which the owner swore was ok last time he chrono'd it - 354 fps, right in the ass hurt like a mofo).
I don't know if there is a velocity cap for markers in backyards. I know there's some form of firearms classification (if gat breachs some form of velocity x mass x impact force calculus), Hotpoint can fill you in on it.