Depends a lot on the location: Where I was there has been more than a few SBR's involved in shootings: But all the owners was careful in the use of force and the civil bits was all non-starters.
Gotta love cell phone camera's.
And even then, if you are in an area where the law and the lawyers work more for the crooks than the good guys, don't bother with the SBR route. Even using a sling press the P556 is still a lot more accurate and capable than a Pistol in a lot of places.
Your milage may vary of course.