Merely a suggestion from an experienced service rifle shooter, I would try a different bullet, especially at only 100yds. Try hornady 75 HPBT. They have not let me down at 100 neither have 69s SMK. at 200 I cant tell difference between 75s and 77s. 300 and out 77 have the edge. As you know what works for one may not work for some one else. I have also noticed as others have mentioned, that you can get 2 barrels from krieger (or anyone else, it doesn't matter) they will not likely behave the same, one will always be better. One thing I have no idea about, and was just wondering myself, velocities out of these carbines, and the effect on accuracy. bullets seem to have a sweet spot in velocity that yields peek accuracy. Maybe what I am asking is what kind of velocities and accuracy do you guys get out of these various length barrels. I know in service rifle a NM AR15 that doesn't shoot 1/2MOA is a tomato stake, and while this is a different "game" what is acceptable? At what point is something wrong?