I won't get "hemmed up" either. Not illegal. I can put my .44 magnum revolver to my shoulder and shoot it if I wish, doesn't make it a rifle. Of that was the case all the people on YouTube shooting these and the AR pistol variants would be in jail.
Chances are in Oklahoma you're right. There could be a chance that you run into a cop having a bad day who doesn't care what the law says, but will arrest by what he sees and you arguing would escalate it... but again not likely in most parts in OK.
With that said, a separate argument... just because it's "legal" or not "illegal" doesn't prevent a cop making it a very bad day and arresting you anyways telling you to prove it to the judge. It's still time and money out of your pocket. Chances are the DA won't prosecute and charges dropped, though.
In Washington state, people have been arrested for using the sig brace as a stock. Police there have been reported to say that they will arrest anyone seen who misuses the sig brace as a stock.
Anyways, enjoy the SB. They're fun to shoot pistols with.