Big difference between a conversion kit and a complete upper for 22LR. The conversion kit will never match the accuracy of an upper built in 22LR or a complete 22LR rifle. BTW, get a Marlin 60 if you want to shoot better than your GF.
BTW, it ain't the bullet diameter that is throwing you off on your AR conversion kit. It is the smooth bore in the 1st inch of the barrel and the 1:9 twist that gets ya (unless you're shooting Aguila SSS 60gr).