Just wondering which part of an AR-15/M-4 is more important, the upper of lower receiver? I am looking at a Norc M-4 to start of with and want to possibly slowly build up and match and mix from there, is this a viable plan? Also anyone have any thoughts on the .22 conversion kits? I want to do the shooting on the cheap till I have my basic level skills up and I am thinking this is pretty viable way to do it. Any info is appreciated.