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The difference between the .223 Remington and 5.56x45 mm

4K views 24 replies 11 participants last post by  brownie 
#1 ·
#2 ·
When shooting 556 through a 223 chamber, check the first two rounds for signs of over pressure. None found, shoot away, the chamber is not too short and will handle it.

Far too many ar's chambers are cut to accept both dimentionally [ like the M1a's are chambered to accept either the 762x51 or 308 ] to discount 556 out of 223 chambers without checking for over pressures to determine a particular 223 ar shouldn't be running 556.
 
#6 ·
Correct, just be aware that many 223 ar's chambers are cut for both to be used, hence you have to determine if there's signs of over pressure in your first couple of rounds of 556 through one. No signs of over pressure and you've got an ar that will handle both.

One thing to remember as well is that the 556 throat being longer, as erosion occurs the 223 could cause problems at that time. Usually around the 5K mark of rds through the barrel.
 
#10 ·
Irregular bulging of the wall I knew was a sign of trouble, but thank you for the clarification. I was concerned when you said evidence of over pressure you might have been referring to something you would see occurring to the upper, maybe in the chamber area. But a flat primer I have not heard about. Tried to find a picture showing normal post shot primer and one showing "flat" but was unsuccessful. Anyone have pics of the two for clarification? Save us 1,000+ words?
 
#18 ·
Here's one that explains it pretty well
_______________________________________________________________________

There always seems to be a lot of confusion over the difference between a .223 and a 5.56 chamber. Is it safe to shoot this? Is it safe to do that? More confusion is added when some manufacturers advertise rifles with .223/5.56 chambers. I found this 'excellent' explaination on Wikipedia (http://en.wikipedia.org/wiki/.223_Remington) and wanted to share it on this forum.

The .223 Remington is a sporting cartridge with the same external dimensions as the 5.56x45mm NATO military cartridge. It is loaded with a .224" diameter, jacketed bullet, with weights ranging from 40 up to 90 grains, though the most common loadings by far are 55 grains.

The primary difference between .223 Remington and 5.56 x 45 mm is that .223 is loaded to lower pressures and velocities compared to 5.56 mm. .223 Remington ammunition can be safely fired in a 5.56 mm chambered gun, but the reverse can be an unsafe combination. The additional pressure created by 5.56 mm ammo will frequently cause over-pressure problems such as difficult extraction, flowing brass, or popped primers, but in extreme cases, could damage or destroy the rifle. Chambers cut to .223 Remington specifications have a shorter leade (throat) area as well as slightly shorter headspace dimensions compared to 5.56 mm "military" chamber specs, which contributes to the pressure issues.

While the 5.56 mm and .223 cartridges are very similar, they are not identical. Military cases are made from thicker brass than commercial cases, which reduces the powder capacity (an important consideration for handloaders), and the NATO specification allows a higher chamber pressure. Test barrels made for 5.56mm NATO measure chamber pressure at the case mouth, as opposed to the SAAMI location. This difference accounts for upwards of 20,000+ psi difference in pressure measurements. That means that advertised pressure of 58,000 psi for 5.56mm NATO, is around 78,000 psi tested in .223 Rem test barrels (SAAMI .223 Rem Proof MAP is 78,500 psi so every 5.56mm round fired is a proof load, very dangerous). The 5.56 mm chambering, known as a NATO or mil-spec chambers, have a longer leade, which is the distance between the mouth of the cartridge and the point at which the rifling engages the bullet. The .223 chambering, known as the "SAAMI chamber", is allowed to have a shorter leade, and is only required to be proof tested to the lower SAAMI chamber pressure. To address these issues, various proprietary chambers exist, such as the Wylde chamber[2] or the Armalite chamber, which are designed to handle both 5.56 mm and .223 equally well.

Using commercial .223 cartridges in a 5.56-chambered rifle should work reliably, but generally will not be as accurate as when fired from a .223-chambered gun due to the excessive leade. [3] Using 5.56 mil-spec cartridges (such as the M855) in a .223-chambered rifle can lead to excessive wear and stress on the rifle and even be unsafe, and the SAAMI recommends against the practice.[4] Some commercial rifles marked as ".223 Remington" are in fact suited for 5.56 mm, such as many commercial AR-15 variants and the Ruger Mini-14, but the manufacturer should always be consulted to verify that this is acceptable before attempting it, and signs of excessive pressure (such as flattening or gas staining of the primers) should be looked for in the initial testing with 5.56 mm ammunition.
 
#21 ·
Next question:

Should I contact smith and Wesson first
, or should I have the store (Shoot Straight) I purchased it from look at it?
That would be my choice, it's their gun and they'll have the proper tools to check head space, etc.
 
#20 ·
its worth noting what causes the primer flattening...the over pressure actually pushes the primer backward out of the case and flattens it against the bolt face...it takes some doing to make that happen...
 
#22 ·
Flatten primers can also appear due to soft cup material as well as loose primer pockets, the latter especially in reloaded ammunition. While flattened primers can be an indicator, they are not always necessarily so.

In bolt rifles, a better sign of overpressure is cases sticking in the chamber during extraction.
 
#24 ·
I have an AR (Century) that had one (and only one) .223 Rem bulge and stick in the chamber... had to cycle the action with a brass punch to get it to extract. took it to my local gun-whisperer who pronounced the gun to be OK..... Bad round?
 
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