If I do the pressure conversion by Bramwell's approximation I get 49.5 cup -> 56,000 psi, give or take. My power curve fit to Bramwell's data, which has a slightly better rē test result than Bramwell's puts it closer to 57,000. So, I'm guessing that the cases capacity used in developing the manual might be a grain of water greater than the one I used? That would still be normal. 55,000 psi is SAAMI's upper limit, but the CIP uses 4300 bar, which is 62,366 psi as an upper limit. You are likely inbetween the two.
I recall hearing the magtech primers have heavier cups than Winchester and Federal? Closer to the CCI #41. That may explain why your commercial rounds have flatter primers? In addition to cup hardness, difference in case headspace insertion length often affects primer appearance, too. In particular, if you put a brand new case in an RCBS Precision Mic or the case comparator on a Hornady (nee, Stoney Point) caliper adapter and measure it, then measure it again after firing and resizing, you find the sizing die never quite puts it all the way back. This means, when the brass is new and the firing pin shoves it forward and the firing primer backs out and pushes it firmly into the headspace determinant (the chamber shoulder in the .223), that primer has backed out further than it will firing that same case after resizing. This inflates the head of the primer more, making it flatter when the case stretches back and reseats it. Bottom line, new brass gets flatter primers, all else being equal. That and cup hardness differences are why primer appearance is not a very reliable pressure indicator by itself.
I have also heard of some folks having some largish MV extreme spread with Magtech primers. Others seem to have no problem. Have you got any chronograph results? I would beg borrow or steal some Federal 205 or 205M primers to compare. My local Gander Mountain had 205's on the shelf last time I looked. They are both mild and consistent. On another thread I recommended to a .221 Fireball shooter that he change from Remington 7 1/2 (a magnum BR primer) to the Federal 205M primer because he was having high extreme spreads. He did, and it cut both the ES and the group size roughly in half for him.
You hear people say Federal primers are too soft for floating firing pin gas guns, but I've never had an issue as long as they were the required 0.003" to 0.005" below flush with the face of the case head. The Sierra manual's section on loading for military gas guns says that was their experience, as well. They had put many thousands of 210M's through Garands and M14/M1A platforms without incident. Federal published a defense against complaints their primers were too sensitive in Precision Shooting in the early 90's, IIRC? They pointed out that while it was true their were more match reload slamfires reported using their primers than with others, it was also true that more match shooters reloaded with their primers than any others, so that was an inevitable consequence. Handloads with high primers are almost always the actual culprit in slamfires. Also, the AR bolt is inherently less prone to slamfires than the Garand and M14 designs, anyway. So I keep using 205M's for .223 reloads. I still use 210M's on the '06 and .308, too, but have gone to #34's where my load doesn't fill the case well. The magnum primer seems to help when there is empty space to pressurize.
Do you have a measured velocity for this load? What I have in mind here is to use Chris Long's optimum barrel time test. If I have your fired case water capacity and your average real velocity, I can tweak the powder burn rate to get a match to your lot. With that, the barrel time can be determined and compared to Long's projected sweet spot times. Might be worthwhile?
Also, have you been measuring the finished cartridge runout you are getting with these bullets? Their ogive shape might not like your seater die?
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