So: what is the practical difference produced by brass height in shotgun shells?

Well, I could just answer “none at all”, and end this article right here.  But then my editor would frown at me and I would indeed provide a poor service if I didn’t explain “why”.

Shotgun shells were initially made of cardboard, with the brass head pressure fitted and glued to the cardboard tube, which contained powder, wad and shot much like an old paper cartridge.

The brass head only function was to provide a gas seal to the chamber.
 

With the introduction of more powerful and haevy loads the added stress of more powder burning longer caused the weak cardboard to separate from the brass at the junction.

Higher brass provided a wider contact surface to the cardboard, eliminating the problem.

With the advent of plastic shells, with the much stronger polymer tube pressure and heat fitted to the head, the problem disappeared… but by then the equation “higher brass = more powerful + more reliable shell” was well established, and manufacturers continued to produce magnum or heavy weight loads with higher brass as a sale point, like fancy packaging and tube coloring, the small additional cost being more than balanced by the much higher retail price.

Higher brass does not “offer better support for higher pressures of magnum loads”, since 1) it’s the chamber walls, not the flimsy metal sheet of the shell head that resist the pressures of the burning powder and 2) magnum loads have the same maximum SAAMI pressure than standard ones.

Also, high brass shells does not offer “better reliability in semiautos”: it’s just that high quality shells undergo tighter quality control checks and that, as said, low brass is usually used in weak clay shooting loads that may not cycle all semiautos. But brass height has nothing to do, per se, with reliability: the difference lies in quality control and load strength.

Most reputable brands’ reloading manuals, such as Lyman’s, specifically state that high or low brass makes no difference, and just internal construction is relevant in terms of loads and performance.