Most NSN (no serial number) guns are inexpensive .22 rifles and the like. Browning, Colt, and S&W have always serial numbered their guns. Winchester on the nicer ones, with the single shot boy’s rifles and the like being NSN. For example I have 2 model 67’s that are NSN, but a 30’s vintage 57 is numbered.
True, and there are many shotguns of various styles and qualities that lack serial numbers, too. I’ve noticed the Winchester pattern myself, with a pre-war 69A Match and a slightly older Model 52.