"Square load"; When the height of the
shot column is equal to bore diameter. In 12 gauge that comes to 1 1/16 oz. of #6 shot.
NOTE: A square load of shot was found to have the most EVEN shot distribution, thus the most efficient. (Fewer spaces or clumps & without using more shot than was necessary.)
In common misuse, it CAN describe another principle of loading gunpowder in shotguns, loading a greater volume of powder than shot is a total waste! In fact by reducing the powder volume, patterns improve, down to about 30% less volume than shot.
My Mother, who just turned 98, says "Nufs as good as a lot". Last August my Son used some of my old BP loads. He used all the good stuff and pulled a few from another box (80 gr FFg under 1 1/8 oz #8) In bright sunlight the flame appeared like a foot long blowtorch bloom. Did the knockdowns fall? Yep. Any faster than the factory superlights? NO!
We pay $30 per pound for GOEX here, to use more than necessary is pure is pure CONSPICUOUS COMBUSTION