Re: Whats the deal with loading for an M1A?
The M1A gas system is designed for a port pressure of 12,500 PSI, measured in CUP back in those days.
Tolerance is +/- 2,000 psi.
Bullet travel is about 12.5 inches to clear the gas port which is about 14-1/8 inches from the breechface.
Cross-referencing some (rare) published data which included both chamber pressure and port pressure on various reloads (I think it was the C.E. Harris 1983 article in American Rifleman), certain loads tested there, and my own current loads and QuickLOAD, they tend to agree on velocity, but pressures look a bit higher--until I remember that QuickLOAD predicts piezo transducer PSI, which often gives a higher NUMBER.
There's no hard and fast correlation or curve to match or formula to "convert" between CUP and piezo PSI, but in .308 using bullets from 150 to 190 grains, it *appears* that QuickLOAD's PSI figures on port pressure are about 1,800 or so higher on the number than the CUP number for identical loads.
Powders no slower than 4064 or Varget peak chamber pressure for every load I've looked at BEFORE the port pressure gets out of hand. I'll have to re-examine RE-15. I tested a great load with that and 175 SMKs, but thought the port pressure was too high based on the predictions. That was when I was not doing any loads more than 14,000 PSI predicted port pressure per QuickLOAD, before I compared with other known data (all using LC cases, BTW).
I've also had three dies and two barrels and NEVER any need to use a small base die.