The square bottom grooves were used because they held more lube and have less tendency to close up because they are deeper. This of course was when most lubes were inefficient; they lubricated with volume. Alox lubes and BP lubes still require large quantities and most designs have square lube grooves. Alox seems to work better with large square grooves. I do like the "classic" look of a square grooved bullet.
The square grooves have one big ailment: they sometimes are difficult in releasing from the mould blocks.
Round grooves drop easier, for the most part, but require better lubes because they hold less and can close up under pressure, which stresses the lube. If the lube can withstand this pressure (hydraulic force) round bottom grooves for me have proven better at higher speeds (2,800 fps) and pressure.
The bullet designs are generally more rigid too.
Round women/round grooves for me!
