Right. They did start with the 222 Rem, and by playing with powders, one of the developers got it to do what the military asked, but they realized they had got it well overpressure in the process of making that happen. The 222 Magnum was a little bigger than they wanted, so this is when they went to Remington to design a case that would give them more room for powder so they could get that same performance with a lower peak pressure. They didn't get the pressure as low as 222 Rem pressures, but they got it down some.
Yes. That's the article I was referring to (but didn't remember where to link to) that has the example of two 5.56 chambers bracketing the pressure produced by the same ammo in a 223 Chamber. Just scroll down below the pressure graphs to the blue bar graphs.
I will argue the pressure in that particular graph you put up is within norms for 223 Rem for two reasons. First, the SAAMI limits can be confusing, and the article uses the wrong one. The number we usually refer to as a maximum is the Maximum Average Pressure or MAP. That is 55,000 psi for 223 Rem as measured by the conformal transducer. But the MAP is only for freshly loaded cartridges. Once a lot# of cartridges is no longer fresh, the limit goes up to a different number called the Maximum Probable Sample Mean (MPSM). For 223 Remington, the MPSM is 58,500 psi. This is the value the M855 in the test should be compared to because it isn't freshly made by the time it gets to the user. Further, the SAAMI allows an extreme spread of 11,300 psi for 223 Remington, but using their 4% pressure SD allowance, the average spread for a sample size of ten would be ±3.078% or 56,699 psi to 60,711 psi. The extreme spread in the graph is half that, and 58,500 psi is still just under the average for that graph, but I will still argue the graph doesn't go over actual SAAMI specs for another reason: The measuring system.
As I mentioned in my post, when a SAAMI conformal transducer reads about 58,100 psi for a 223 Rem, a CIP Kistler transducer will read 62,366 psi. The SAAMI-type conformal transducers are odd with the 223 Rem. All the other rifle cartridges that have 52,000 CUP ratings read up in the 60-65,000 psi range on the conformal transducer, except the .223 Rem, it is down at 55,000 psi. Odd, but not the only cartridge with a narrow cylindrical wall it reads low with. The strain gauge system used in the article author's tests reads up where the CIP system does, being better on absolute value (assuming proper calibration). So it is expected to read around 7⅓% higher than the conformal transducer does with 223 Rem, as demonstrated in Dr. Brownell's work. So the 60,050 psi-ish average pressure on that graph would be expected to read an average of just under 56,000 psi on a conformal transducer gun, putting it within the MPSM SAAMI has for the maximum average value of a not-new lot, and that's why I don't think that graph actually represents being out of SAAMI spec for a no-longer-fresh box of ammo.
The last plot in the article is another matter. That has a SAAMI-type V&P barrel reading M855, 10% higher than it reads in a 5.56 chamber. It's showing a difference of about 9,000 psi, which can happen if you pick the right combination of 5.56 reamer and 223 reamer to cut the chambers. When the Army started enlarging parts of the M16 chamber for reliability, they only reported a 2,000 psi drop in peak pressure. If you use the GRT software's freebore calculator, you see that the 2,000 psi number agrees very well with the substitution of a 0.0566" 5.56 NATO freebore (PTG and JGS 5.56 NATO reamer dimensions) for a 0.025" 223 Rem freebore. But this is where it gets fuzzy. If you look at 5.56 reamer dimensions from Clymer and JGS and PTG, you find the Clymer 5.56 reamer is diameters are different from those of its 223 reamer, while the others are different pretty much only in neck diameter and freebore. A case expanded under pressure to the Clymer chamber dimensions will have almost 2 grains more water capacity at the pressure peak than one expanded into either a Clymer 223 chamber or a 5.56 chamber made using the JGS or PTG reamers. That added volume is responsible for about 8,000 psi drop on top of the drop due to the longer freebore. For the JGS and PTG reamer dimensions, the pressure difference is almost entirely due to the freebore and a little bit to the wider neck allowing a bit more propellant gas to bypass the bullet before it obturates the bore, and hence they only see about a 2,000 psi difference.
I don't have a 5.56 NATO chamber drawing to compare to the different reamers, and one would be welcome if someone has it.
JohnKSa said:
Yes. That's the article I was referring to (but didn't remember where to link to) that has the example of two 5.56 chambers bracketing the pressure produced by the same ammo in a 223 Chamber. Just scroll down below the pressure graphs to the blue bar graphs.
I will argue the pressure in that particular graph you put up is within norms for 223 Rem for two reasons. First, the SAAMI limits can be confusing, and the article uses the wrong one. The number we usually refer to as a maximum is the Maximum Average Pressure or MAP. That is 55,000 psi for 223 Rem as measured by the conformal transducer. But the MAP is only for freshly loaded cartridges. Once a lot# of cartridges is no longer fresh, the limit goes up to a different number called the Maximum Probable Sample Mean (MPSM). For 223 Remington, the MPSM is 58,500 psi. This is the value the M855 in the test should be compared to because it isn't freshly made by the time it gets to the user. Further, the SAAMI allows an extreme spread of 11,300 psi for 223 Remington, but using their 4% pressure SD allowance, the average spread for a sample size of ten would be ±3.078% or 56,699 psi to 60,711 psi. The extreme spread in the graph is half that, and 58,500 psi is still just under the average for that graph, but I will still argue the graph doesn't go over actual SAAMI specs for another reason: The measuring system.
As I mentioned in my post, when a SAAMI conformal transducer reads about 58,100 psi for a 223 Rem, a CIP Kistler transducer will read 62,366 psi. The SAAMI-type conformal transducers are odd with the 223 Rem. All the other rifle cartridges that have 52,000 CUP ratings read up in the 60-65,000 psi range on the conformal transducer, except the .223 Rem, it is down at 55,000 psi. Odd, but not the only cartridge with a narrow cylindrical wall it reads low with. The strain gauge system used in the article author's tests reads up where the CIP system does, being better on absolute value (assuming proper calibration). So it is expected to read around 7⅓% higher than the conformal transducer does with 223 Rem, as demonstrated in Dr. Brownell's work. So the 60,050 psi-ish average pressure on that graph would be expected to read an average of just under 56,000 psi on a conformal transducer gun, putting it within the MPSM SAAMI has for the maximum average value of a not-new lot, and that's why I don't think that graph actually represents being out of SAAMI spec for a no-longer-fresh box of ammo.
The last plot in the article is another matter. That has a SAAMI-type V&P barrel reading M855, 10% higher than it reads in a 5.56 chamber. It's showing a difference of about 9,000 psi, which can happen if you pick the right combination of 5.56 reamer and 223 reamer to cut the chambers. When the Army started enlarging parts of the M16 chamber for reliability, they only reported a 2,000 psi drop in peak pressure. If you use the GRT software's freebore calculator, you see that the 2,000 psi number agrees very well with the substitution of a 0.0566" 5.56 NATO freebore (PTG and JGS 5.56 NATO reamer dimensions) for a 0.025" 223 Rem freebore. But this is where it gets fuzzy. If you look at 5.56 reamer dimensions from Clymer and JGS and PTG, you find the Clymer 5.56 reamer is diameters are different from those of its 223 reamer, while the others are different pretty much only in neck diameter and freebore. A case expanded under pressure to the Clymer chamber dimensions will have almost 2 grains more water capacity at the pressure peak than one expanded into either a Clymer 223 chamber or a 5.56 chamber made using the JGS or PTG reamers. That added volume is responsible for about 8,000 psi drop on top of the drop due to the longer freebore. For the JGS and PTG reamer dimensions, the pressure difference is almost entirely due to the freebore and a little bit to the wider neck allowing a bit more propellant gas to bypass the bullet before it obturates the bore, and hence they only see about a 2,000 psi difference.
I don't have a 5.56 NATO chamber drawing to compare to the different reamers, and one would be welcome if someone has it.
