My sps tactical 20" doesn't really care what you feed it in terms of powder as long as the burn rate is in the appropriate range. Brass prep and neck tension seem to be the keys to good loads. FGMM gets really hard to resize and the primer pockets get kinda loose after 3 or 4 loadings. PMC brass is super hard in my IMHO plus the flash holes are never on center. I like stuff thans annealed from the factory.
I get sub moa groups with all of the following 168gr loads:
168 SMK loaded .020 off the lands (in my remington thats 2.885, yes 2.885)
168 AMAX loaded to 2.915
168 Hornady Match loaded to 2.900
LC or Federal Nato Brass and sometimes Federal GMM OR R.C. (Primer Pockets uniformed, Full length sized, trimmed to 2.004 - 2.006) Federal 210M, CCI BR2, W/W or just about any primer
39.5 gr IMR 3031
42.5 gr Varget
42 gr of RL-15
41.5 gr Bartlett surplus 4895, IMR 4895 or H4895
42.3 gr N140
I've tried them all, the most consistant have been Varget and Rl-15 but its definately a push, I don't shoot paper further out than about 200yds so the difference in any of these powders is neglegable to me. All listed loads are suggested loads from Zedikers or the Sierra Book developed years ago by guys who shoot alot more than I do so I figure why go hotter, why beat up the chamber or the brass. Another upside is The loads are moderate so you don't have to worry too much about some cases having smaller capacity (Lake City and PMC are the thickest as far as I can tell)...
My rifle is sensitive to seating depth, I used to load everything to 2.825 so it could go in the M1a mags too but now I keep that brass separate.
My best groups ever were under 1/2 when I ran out of match kings and finished a flat with some 168 Amax s. I loaded up about 25 before I realized that I forgot to reset the seating die and the OAL ended up at 2.915. when I compared the two different rounds I noticed that the Ogive on the hornady is way further back. I tried to chamber one and the bolt closed with 0 effort so I single fed those and gave it a shot. 5 shots, ONE RAGGED HOLE @ 100yds.
We used the RCBS precision mic to measure my chamber, using a 168 match king, I have to load 2.905 to touch the lands, amaxes have to be 2.935 and hornady match go 2.920.
I hope I'm not rambling on too much, sorry....