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Beware of recently made LC 5.56mm ammo that is being sold on the civilian market. In over 40 years I never found a significant difference in cartridge case dimensions between .223 ammo and military 5.56mm ammo.
Yesterday I measured some recently acquired ammo sold by Federal as XM 193. I was told by a distributor that this stuff is military reject. Every case that I measured had a long neck. Those case necks were .004-.037 longer than military spec. Those case necks are also longer than the necks of 5.56mm ammo made during the 60s and 70s.
A case neck that is .004 longer than spec would most likely not be a big deal. A cartridge with a case neck that is .037 longer than spec could could see the case mouth jammed hard into the throat. In this event, a serious spike in pressure could result.
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