While a short barrel rifle does constitute a Title II firearm according to the rules set forth in the National Firearms Act, they are perfectly legal in many states so long as you fill out the paperwork and pay the $200 tax.
IIRC it is ATF Form 4 that needs to be filled out.
If you do not follow the rules, 10 years fed prison+$250,000 fine. Oh, and the ATF tends to carry out such warrants with extreme enthusiasm to the point of kicking in doors, suffocating children with tear gas, and setting them on fire.
You really should just fill out the paperwork.