As long as it is 26" overall length & had a butt stock, Justin, the barrel can be any length - "as mfgr'd".
Witness some modern rifles & shotguns with 12" to 16" barrels, are not restricted here.
Now, to take one manufactured at even 19", and shorten it to less than 18", would be a crime. Go figur,
if a cap lock or modern gun.
About flintlocks, yes - any and all are considered antiques = non-firearms - except for handguns, of course.
Only antique flintlocks handguns are considered to be non-fireams. At least that is how I understand "things".