Any firearm orginally build as a rifle must have a min. barrel length of 16 inches to be legal (varies in some states), however one additional reg. is that it also have a overall length of 26.5 inches. So provided that you meet both of those specs you can put a pistol grip on the gun. Make darn sure that shotgun barrels are 18+ inches and overall length of 26inches, with the exception of a RIFLED 410/45 long colt barrel (must be rifled) can be 16 inchs!!!!
There are very few exceptions to this rule (, some AW rifles with folding stocks have overall length shorter than 26.5 closed but must have an attached buttstock which brings the overall length to over 26.5 inches when open and some firearms are neither rifle or pistols,, along with certain C&R guns)
encore and contender pistols were originally built as pistol frames and can be switched between pistol and rifles....