PUMA 16" Barrel is it legal?

i ran into that issue with my short barreled m-4. ignorance of the law isnt an excuse as the atf man knew my barrel wasnt 16", it was 11.5 and i didnt hav it registered and he told me to take it out of the store ( where i was getting a 4 rail free float handgaurd installed) disassymble it and olny bring the upper portion back in and tell him it was for a pistol. i did as i was told so as not to get it confiscated and a big fine. the reads the rifle barrel has to be ATLEAST 16" to be leagal. im no lawyer but if it was me, i would keep carrying the rifle and have the tx and federal law printed out and if that same LEO pulls u over just to ck u, and givs u crap about it again, u can show him the law and inform him to have a nice day and then ask for his supervisor to be called out to the location if the cop continues to want to be a bur under a saddle. he cant make up the law as he goes just to satisfie himself. use the law against him and he wont have a leg to stand on