I remember reading that the splines on most USGI flash suppressors are squared at the ends, versus the rounded splines on commercial ones. I'll try to dig up a photo of my USGI flash suppressor, which my government representatives have deemed too deadly for use on my M14 rifle.
Military flash suppressors may be made from forgings or cast. Most are forged. The splines that align the suppressor to the barrel are always square on the ends. Flutes may be rounded on the ends or squared. The muzzle end will almost always be radiused or beveled on the outside diameter.
Commercial suppressors are usually made by casting. The splines will usually be round on the ends although some may have square ends. The flutes are often much wider than those on GI suppressors and the muzzle end will have a much sharper edge. Most commercial suppressors are made with a larger inside diameter to simulate the reamed suppressors used on National Match rifles.
Early GI suppressors were cast. The failure rate for cast suppressors was too high so later production was done by forging. I should note however that in my 35 years of association with the M14 rifle, I have only seen two or three flash suppressors fail. Failures are usually caused by gas cutting that leads to one of the splines breaking away at the rear or front of one of the flutes.
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