Dont you know how it works in real?? You are allowed to wear the weapon, but not have it visible. I think USA is the only country where this is allowed.
I would think basically this rule is correct.
But if you Frisk somebody and find a Gun which is not buyable from Ammunation, i'd suggest arresting him or revoking at least the gun and warn him orally.
If you carry a legal gun which is buyable in Ammu, and u get frisked and its found, you can't be punised for it (as long as u got weapon license)
But as M4 and mp5 are concerned to be military rifles, even s.b. with license shoul't have them, thats my opinion and would make the whole
**** more realistic. In ammunation there only are guns for Civilian self-defence or hunting sports. And only they are Legal. (this is the most realistic way)