I have a mossberg with an 18.5" barrel. With the pistol grip on it, its just over the 26" length. ( dont remember exactely, but about 1/4" to spare).
Now if you subtract 4.5" off my barrel, then there is no mathematical way to stay above 26".
You'd have to fond a stock with a pistol grip.
I think if you re measure your Mossberg you will find it quite a bit longer then 26 1/4 inches with a pistol grip.
From what I've gleaned from the posts of those "in the know", I believe that the 26.25" minimum is only applicable to shotguns that are sold without a standard stock, or if a foldind or telescoping stock has been fitted.
* A firearm that can fire after being reduced, by folding, telescoping or otherwise, to an overall length of less than 660 mm;



























