A good carbine will shoot 3-4" groups at 100 yds
The fact that they have to be moved all over the place to center them up is because they were built in a very hasty manner and the sum total of little errors adds up to crooked barrels, misaligned receiver holes, badly indexed barrels etc. The M1 Garands are little better and they put the FRONT sight in an adjustable slot to make up for this stuff. I have an original 5.6 Winchester that is in just wonderful condition and unaltered since built. When viewed from the rear sight area, nothing lines up, everything is crooked. Yet it shoots spot on to POA at 100yds. So we have an expensive original, by the company that made guns punching out a pretty shoddy product from a cosmetic standpoint that works perfectly. Now take that same situation and add a sight that it never had before slapped on in haste and what's left?