Everything I have read about the flash hiders for the M1 is, they do nothing, except possibly reduce accuracy.
But they look cool.
Somebody did a comparison relatively recently maybe the GCA using the m2 flash hider with 173 gr ball ammo if I recall correctly.
It was more than one rifle selected and a few exhibited better groups during the testing.
More than likely it was just better stock or handguard fit or some other circumstance which had an additive effect. I cant remember the exact conclusions but it was an interesting read at the time.
