The reason you should not use commerical ammo in a Garand without an adjustable gas plug is becasue the military powder burned faster, creating less pressure, and a shorter explosion. The commercial powder in todays cartriges uses slower burning powder, creating high pressures, and in some instances, the burning of powder almost half way up the barrel. This can cause an ossilation of pressure, that really screws with the op rod gas system. Of course, this is identical round to round, so it does not affect accuracy, but it can seriously damage the op rod.
And I have checked, a clip IS NOT considered a magazine, because it is ejected automatically when it is empty, it is fully inserted into the firearm, and it does not contain a spring.
Just a young guy that likes old cars and straight shooting guns