Here is the brass I have used in my 20P's in order of my preference and comments:
Lapua - the most accurate brass but loaded necks measure 0.2305" and I am shooting in 0.233" neck chambers
Old IMI brass - available from Wideners in the late 80's, great brass that lasts forever
FC Military brass - BrassBombers only fired in rifles (none in SAW's), very tight primer pockets, crimp slightly different (thicker necks)
IMI 15 - purchased as loaded XM193 and fired in winter
LC 09- purchased new so no crimp, never fired with 5.56 higher pressure loads
LC 17- purchased as loaded XM193 and fired in the winter
LC - purchased as once fired military brass from BrassBombers, depending on the temperature it was fired, can have some with loose primer pockets
Starline - I haven't fired these yet (thicker necks) might be rated higher
Winchester - I haven't fired these yet, might be rated higher
YMMV
