.223 and 5.56 operate at about the same pressure as the NATO maximum pressure of 62,000 PSI works out to be about the same as the SAAMI 52,000 CUP. The only appreciable difference between SAAMI and NATO chamber specs is the amount of freebore. This is what can potentially cause the pressure spike, using heavier bullets in NATO cartridges. It has nothing to do with NATO and SAAMI spec cartridges operating at different pressures, as they don't. Now, most manufacturers use modified SAAMI chambers, as the fast rate of twist, which allows for heavier, longer bullets to be used, necessitates an increase in the amount of freebore in the chambers. The Savage Axis has a 1 in 9 in twist rate, meaning it's designed to accommodate heavier bullets, up to about 68 grains (M855 contains 62gr projectiles). So theoretically, there should be no issues with firing M855 out of his rifle, but I was suggesting he try a few rounds to verify that there isn't an unsafe pressure spike before shooting large quantities of it through his rifle.
The leade angle is also different and probably the more important difference. Bullet weight has nothing to do with it either. The chambers were different even before the NATO load moved to the 62gn M855 bullet. Even M193 will cause high pressures in a .223 chamber and it is a 55gn bullet. The 5.56 has always been loaded hotter and has always needed the longer freebore and shallower leade to keep pressures within limits.
Don't you find it odd that the .270 Win is specd for 52,000 ANSI CUP and 65,000 ANSI PSI, but .223 Rem is specd for 52,000 ANSI CUP and 55,000 ANSI PSI?
Look at the CIP standards which use the same measuring method as NATO. They specify the exact same pressure for the .223 and the 5.56. Again the difference is that the chamber of the 5.56 allows a cartridge with more powder behind the bullet to stay within that pressure limit. That is the only point that really matters in the discussion. The case dimensions are the same. The cases are the same. The bullets are the same. The rifle can handle the pressure of the 5.56 round, because if the rifle is produced or sold in Europe, that is the pressure of commercial loaded ammo in the proper chamber. The problem is that 5.56 ammo in the .223 chamber generates MORE pressure than the 5.56 round does in a 5.56 chamber. Tested into the 85kpsi range kind of more. That's proof load territory.
I stand corrected but the fact remains that the pressure number is the same. Pressure should be the same throughout the vessel and peak pressure occurs after the bullet has left the case. Whether the transducer is in the neck or body of the case is splitting hairs.
For what it is meant to measure, it's truly terrible. When it was all that there was, it was better than nothing, but not by much. Now that there are methods for measuring pressures directly, all references to CUP need to be eliminated. I can get about as accurate a measurement of length by looking at something as a copper crusher gives of peak pressure...actually I can probably eyeball it better. I wouldn't trust my TLAR with my safety.
So why is this?
