A real inch pattern geek question..

[Flying10uk]

A square corner is easier/quicker to machine than a radius corner, especially if carried out on a manual machine or, when FN/SLR rifles were produced, CNC machines were in their infancy.

Sometimes people think that you can "just stick a rad on a corner" or "machine a square hole, with square corners using a round cutter that is revolving", no problem. Sometimes things are not that simple and more complicated machining equates to more time/cost.