Before you order or at least receive those inserts you might want to check the locations of their O-rings. That is if they are the Chaszel inserts. Those inserts use O-rings around the insert to center them in the shotgun bore so if the muzzle end O-ring is within 6" of the muzzle you will be cutting it off or it will be sticking out of the bore.
Okay then I just looked and yes, the O-ring grooves are near the muzzle so you might want to cancel and re-order 18" inserts or be prepared to have new O-Ring groove(s) machined or use a sleeve of some sort.
https://i.postimg.cc/HnFFZzXp/12ga-4...h-Overall.webp
Longbow