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Posted: 11/10/2006 12:33:13 PM EDT
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I just got a used DPMS A4 upper. It is marked DPMS 223 1/9. My question is I have heard DPMS marked all their earlier barrels with 223 when in fact the upper is chamber in 5.56. Is this true? Can I fire 5.56 in this upper? Thanks for any info. |
| This is just my first post of hopefully many to come. I thought that there was a difference between the two. I have been having this same debate and is why I finally joined. If I recall, the difference is that the 5.56 is hotter than the 223. I was always told that you could fire the 223 through a 5.56, but not the other way around. Something about the shoulder or throat in the barrel, but I do not really know and I would really like to know for sure too, because I am thinking about getting a bull barreled upper that is marked for 223, but most of what I shoot is 5.56 . |
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Both .223 Remington and 5.56mm NATO are loaded to a maximum of 52,000 psi. However the SAAMI spec .223 chamber is a little smaller in the neck and both the leade and throat are a little shorter than in a 5.56mm NATO chamber. What this means according to SAAMI is that shooting 5.56mm ammunition in a .223 chamber theoretically could result in chamber pressures a little higher than normal. This could potentially push the max pressure to around 55,000 psi. However, in practice I have never seen any signs of excess pressure when firing 5.56mm US govt surplus ammunition in a .223 chamber other than perhaps the occassional mildly flattened primer in some rifles. There are however other variables in chamber pressure, one of the most significant being temperature, so your mileage may vary, particularly in hot weather or with a round that has heat soaked in the chamber. The potential problem really takes care of itself however as the shorter and/or lighter barrelled military styled AR's whose users would probably be more likely to use mil surp ammo tend to be chambered in 5.56mm, while the heavy barrelled rifles more likley to be used by varmint hunters, who are not likely to use less accurate mil surp ammunition, tend to be chambered in .223. However unless you are putting a steady diet of 5.56mm through a .223, it is most likely not going to cause any disasterous consequences in an AR as the rifles are proofed to 70,000 psi, so even 55,000 psi is still well within the proof limits. If the pressure from firing a given 5.56mm load in your particular rifle is excessive it will be evident in the form of flattened primers (where the otherwise nicely rounded shoulder of the fired primer is virtually non existant due to the primer being forced back against the bolt face during firing) or even cratered primers (where the primer is flattened but where also the metal around the edge of the firing pin indentation in the primer begins to flow back into the firing pin recess.) The next step in the excessive pressure progression would be expanded primer pockets causing loose primers which usually co-occurs with expanded case heads that do not return to their original dimensions. All of these signs will occur before any more serious case failure occurs - at least in new brass. |
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