AR-15 Bolt Carrier group...

[RLHas]

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!

[Constructor]

I would run parts made by a DOD contractor before some of the stuff made for commercial sales. CMT/Stag, Colt if you can get them, I think BCM and I get them from the same contractor. Haven't tested a LMT but pretty sure they are fine.
You should check the headspace, too much will flatten primers or cause head seperation if bad enough, too little will just cause the cases to stick in the chamber.