Some allow their LEO to take their duty full capacity mags home and use them for off duty practice, such as IPSC matches, if the officer has a Rpal and compatible firearm.
Well if that's happening , He is breaking the law. Competing with his personal gun don't allow him to use a prohibited mag in it, just because he can carry on while on duty.
there are some 40s-w cal mags that will work in 9mm pistols with that will carry more them 10 rds and that is legal.