400 yards is reaching the upper limit of the ability of the .223 -L (some may argue against that with good results and have reached much further). Where as 400 yards is reaching mid range for the -M. Of course this is talking for paper or gong punching, if you're thinking for hunting that's a different matter and range would depend on type of game desired. There are some on here, and that I shoot with, that stretch the -M out to 800 (shooting paper and gongs) with decent results.
Gas system adjustment is slightly different for the -M, but still adjustable externally from the front block and brain dead easy. The M has a longer upper, and I believe is heavier than the -L (too busy to Google that right now, but it sure as hell feels heavier). Both are still manageable rifles to haul around. As for accuracy they're about the same, not precision rifles but capable of minute of game animal all day long.
