From what a couple ATF agents have explained to me , it is legal to put a shoulder stock on an Encore frame that is registered as a pistol as long as the barrel is over 16" on a rifle (18" on a shot

their is also a minimum over all length that the weapon must meet (I can't remember but I think its about 32")
BUT I was told that it is NOT legal also know as Illegal to put a barrel shorter than 16" on a rifle frame unless its registered as a class 3 SBR , the question on the pistol grip was explained as it diden't matter what the but was either short rifle stock or pistol grip as long as the barrel made the weapon long enought to meet the minimum over all length requirements
Their it is clear as mud , I would recomend that you call the ATF and ask then have them direct you to the law so it can be read and printed , I've found that alot of times you will get differant answers from differant agents