Since BL-C(2) is a spherical powder, have you deburred your flash holes? I found AA 2520 to be surprisingly sensitive to that, and now do it to all cases I'm loading with a spherical propellant or any other fine grain powder.

Are you seating the bullets out long? At standard SAAMI length, QuickLOAD gives me about 58,000 to 68,000 psi from your load depending whether I use commercial or military case capacities, tight chamber or looser chamber. Potentially pretty warm. Groups opening up are a common pressure sign.

Take some of your fired, but not yet resized and decapped cases and weigh them to find an average one. Record its weight with the spent primer still in it. Fill it with water just flush with the mouth-no meniscus. Tap any air bubbles out, and with it level and gas-free, weigh it again. The difference is the fireformed case water capacity in grains, and that is what affects peak pressure. That's what the software needs to estimate pressure more accurately.

What is the length of the barrel on your gun?

I have run the 77 grain Sierra MK's in my 7 1/2" twist barrel with 23.8 grains of Varget with good success. Varget doesn't seem to care whether the flash hole is deburred or not. That is a compressed load. This is my service rifle match gun, so the barrel is 20". That stubby 77 grain bullet is seated to 2.260" for magazine feeding, which is why it is compressed. Winchester cases. Federal 205M primer. This has been a good 300 yard rapid fire load for me.

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Nick

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"First contemplation of the problems of Interior Ballistics gives the impression that they should yield rather easily to relatively simple methods of analysis. Further study shows the subject to be of almost unbelievable complexity." Homer Powley