My goto M1A (22") load was 175SMK at 2.815" OAL, 42.2gr of IMR-4064, Large Rifle Match Primer, Commercial 308 brass (Win/Rem).
No longer own the M1A, but I use this same load in a Savage 11VT .308 (24", 1:10", heavy barrel). It was a definite Supersonic arrival at 1000yd load with the M1A. With the current 4350ft ASL local altitude and barrel, it's likely supersonic to quite a bit further than 1Kyd.
Like others, I have been trying the 178 ELD-X, and like them, have been getting a lot of pressure. Not there yet. Current brass choices are Starline and PPU.
Have also gotten some decent accuracy from IMI 168 Semi-Auto match but have not taken it past 100yd yet. Been playing with varmint weight bullets too, which stay supersonic further out at the local altitude; they shoot flat and provide expansion.
Greg