I would shy away from QC-10, I know Russell that owns Quarter Circle and have worked with him when I was at SAR for R&D on his lowers. They still have problems and I have probably fixed 40 since they have come out.
Between that and QC10 barrels not being ramped properly, the main thing is the linkage system that controls the last round BHO. IF you don't care about a BHO then its great. If you want a BHO its a serious pain to get it to work correctly on the majority of the lowers. They are still working to fix this.
Do you mean bolts not ramped properly?