Just because an idea is old does not mean it is antiquated. The concept of square load is not based on theory but theorem. It refers "only" to the load: length equals width. (Height is the term used to describe the high point of projectile travel.)
The square load concept is entirely true because it is a basic starting point for ballistic efficiency. The shot load is acted upon as one entity and for all practical purposes is considered one projectile. For it to perform best, the center of mass has to equal the center of gravity. It is therefore a point found by the intersection of diagonals of equal length, which must be found in the square. If a shot charge performs better or worse than the square load for any given gauge, it is because there are external variables applied.
By example, I could relate that I once had a cheap 16 gauge patent shotgun which was the best long range gun that I ever used. In it I shot the cheapest Federal 2.5 dr./eq. 1 oz. Game Loads, reputed to have very soft shot. On doves, the gun was deadly to 85 yards. Was the square load, the secret of success? No. There were other factors at work. The barrel on that cheap gun was made under the supervision of Harry Pope, reputed to be the best rifle smith of the early 20th Century. And the shells were loaded with slow powder. When Federal substituted hotter powder a few years later, the gun started dusting birds. All of these are external variables. But in shotgun ballistics a square load is always an optimum starting point.
