I would say that its legal, a bigger problem though is going to be
keeping the thing on there. (the clamp on jobs have a tendency to
loosen on their own and then end up getting blown downrange! )
Also, unless its designed well, I doubt it will do much to .223. I've
found many brakes on .223 AR's to be nearly worthless... many of them
just increase the noise and blast around the shooter and don't
do a heck of a lot more. At least in this case if thats what happens
you can just remove it.
-Mike