Toward the beginning of the year I had no intentions of ever owning an AR. I had an opportunity to buy an inexpensive Lower assy. and it's snowballed from there. Now I have two complete rifles and another upper waiting for the right lower deal to come by. My question(s) is/are... What concerns should I have about Bolts, Bolt Carriers and changing them out? Is there a certain head spacing procedure I should follow? Is "interchangability" an issue? These are all high quality Mil Spec parts. I just want them to run and run well and I know bolt issues can be common. Thanx for any responses!