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Posted: 9/20/2012 12:08:42 PM EDT
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I built a DPMS pattern .308 AR and am seeing pressure signs on the shell casings. Specifically, the primers are somewhat flattened and cratered/raised around where the firing pin struck, indicating that metal flowed into the hole around the firing pin. There is also a shiny spot where the extractor was.
About the build, it is a DPMS pattern with the following: Fulton Armory 20" match SS barrel with a Fulton bolt-carrier that was headspaced at Fulton with a Fulton free-float quadrail, gas block, and gas tube. The upper RX and internal parts kit are DPMS. The lower is CMMG. It has a Enidine/Magpul PRS/CS buffer spring and buffer tube for buttstock. Ammo: Federal 7.62 175 grain Sierra Matchking and M118LR Observations: The headspace is very tight. I measured the ammo COL of both types of ammo and they are about .05 over. I also measured the base of the spent casing and they are also mic-ing .015 over a unused round. I am thinking that this could be a combination of either tight headspace, long COL, load, but am not sure. Has anyone else seen anything like this? Does anyone have any ideas? |
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Quoted:
I built a DPMS pattern .308 AR and am seeing pressure signs on the shell casings. Specifically, the primers are somewhat flattened and cratered/raised around where the firing pin struck, indicating that metal flowed into the hole around the firing pin. There is also a shiny spot where the extractor was. About the build, it is a DPMS pattern with the following: Fulton Armory 20" match SS barrel with a Fulton bolt-carrier that was headspaced at Fulton with a Fulton free-float quadrail, gas block, and gas tube. The upper RX and internal parts kit are DPMS. The lower is CMMG. It has a Enidine/Magpul PRS/CS buffer spring and buffer tube for buttstock. Ammo: Federal 7.62 175 grain Sierra Matchking and M118LR Observations: The headspace is very tight. I measured the ammo COL of both types of ammo and they are about .05 over. I also measured the base of the spent casing and they are also mic-ing .015 over a unused round. I am thinking that this could be a combination of either tight headspace, long COL, load, but am not sure. Has anyone else seen anything like this? Does anyone have any ideas? More than likely, there is not a headspace issue. It is not unusual to see cratering and flattening of primers in a 308. This is caused by the primer backing out after ignition and the cratering is caused by material being pushed up around the firing pin and then into the firing pin hole. Also, it is normal for spent cases to have a larger OAL than an unfired case. Headspace is generally tighter in a 308 to improve accuracy. |
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