Why counting gun barrels would create another treaty loophole

Would limiting the number of gun barrels have been a better treaty method than limiting ship tonnage?

No. Navies could design convertible twin turrets for a wartime third gun, and ships could grow enormous if displacement itself were not limited.

The proposed barrel-count limit creates another loophole. A navy could fit unusually wide twin-gun turrets with the foundations and internal space for a center gun, then convert them when war approached. It might trade ammunition capacity per barrel to keep the total magazine load similar while expanding from eight guns to twelve. Without a displacement ceiling, the hulls needed to support such mountings could also become colossal. An arms-control metric can quickly become a design target rather than a durable limit.