Since the length is 5.5 inches and 4 inches cover your barrel, your barrel will be 18.5" if this is pinned.
The first answer is wrong for anyone else wondering about this. Visit the manufacturers website, in the product info they have a picture showing specifications and fitment for this product. Manufacturer states this model will add 1.5 inches to the length of the barrel, the other 4" of this is a shroud that goes under the hand guard. The other models add 1" or 1.375" of length to the barrel.