If you only plan on shooting <200 yards you do not need a BDC. You can then zero at 50yards, and have about 1.8" high at 100 yards, and about 0.5" low at 200 yards. Thus, very flat to your max range.
If you really want to know the fine details of 6.8 vs 5.56 you need to pull up a ballistic program and try out different weight bullets. I suppose the smart ass answer is.. -9.8m/s^2.