next time you wonder what exactly is the difference between .223 rem and 5.56 nato the answer is 12,000 psi. .223 rem is rated at about 50,000 psi and 5.56 nato is rated at about 62,000 psi. i think there also might be some very slight chamber dimentional differences.i very much like the .223-5.56 cartridge. it is easy to shoot, accurate, and effective.it is also economical, unlike .338 lapua which costs about 5.00 dollars a round to shoot.
SAAMI 223 max is 55,000 psi. SAAMI 5.56x45 data I cannot find.
CIP (European proof standard) Piezo electric device, spec is 55,000psi for the .223 and 62.000psi for 5.56.
I believe the cartridge cases are the same dimensionally, the difference is the throat or leade. In 5.56 marked barrels the leade is longer allowing 62-75 projectiles to be used without causing dangerous pressure spikes.
But yea Sammi does not have any specs for the 5.56, so in a sense it is the most popular wildcat round in the world. Joking aside, they are more or less the same, but 5.56 Lake City cartridges are said to have a little more volume than 223 remmington cartridges. Which can be a big deal if your reloading. Something I am starting to get into. It is interesting to note that they publish which manufacture they use for the reload data, so it seems to me that it's more manufacture dependent than specs since everyone's specs for the 223 wylde is different by a little bit.
Not only is the neck slightly different, the brass is thicker, allowing for the 9K PSI difference. If you reload with XBR, you have essentially the same as NATO. There's a Swiss powder used by the Swiss for their ammo, on which the SIGs are tested. It's available sometimes here in the US.
There are a few different reamers that gunsmiths can use to make a .223/5.56 shoot. As I recall,a Wylde chamber allows a slightly longer throat. A barrel maker I know uses different bore buttons also. I believe the USMU and the All Guard team has their own reamer. (anyone could use it, I suppose) There was a study of all .223 brass with their different capacities that was posted on 6BR.com. http://www.6mmbr.com/223Rem.html
I wouldn't bet my life on this, but I've heard the pressures are pretty similar, and that difference comes from the methods they use to measure the pressure - SAAMI uses CUP, which isn't exactly similar to PSI, and on one the pressure is taken from the case wall, and the other the case neck. It's pretty hard to compare the two accurately.
My chamber's designed to shoot both, though, so I don't have to sweat it.
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