This could be brass that has been fired in a M60 machine gun. The brass can stretch lengthwise and expand at the base when fired in a machine gun. . Measure the head diameter and check that with the head diameter of the cases you have been using. If you buy USGI brass in the future, try to determine wether the brass has been fired in a rifle or machine gun.
Another item you might want to acquire is a cartridge headspace gauge. Wilson makes these. It measures the headspace of the cartridge case.
Doug
Another item you might want to acquire is a cartridge headspace gauge. Wilson makes these. It measures the headspace of the cartridge case.
Doug