Hi all, I want to put together another weapon from scratch. (I did a kit and it works awsome) I'm told (by a co-worker) that I will not be able to head space it because headspacing is a matter of finding a bolt that happens to properly fit. I'm sceptical of this info due to seeing so many upper builds, and also some saying there is no need to headspace. Is it a matter of bying a barrel, carrier group, and an upper reciever, and such and putting them together or am I missing alot. Any and all input is appreciated.  

+1. What Knee Dragger said .

Thanks fellas, this is looking alot more optimistic.

One more question; will headspacing or anything elso I should worry about change if I swap out an A2 upper for an A3? Thanks again.  

Quoted: One more question; will headspacing or anything elso I should worry about change if I swap out an A2 upper for an A3? Thanks again.

No.  Headspace is based on the chamber, barrel extension, and bolt.  The upper receiver doesn't affect this relationship.  

What about feeramps and stuff like that...about cuts?   Do I need to get anything specific ?  

If your getting a receiver with feed ramps then get a barrel with feed ramp cuts.
Receivers without feed ramps will work fine with barrels that have feed ramp cuts.
Receivers with feed ramps will not work with barrels that do not have feed ramp cuts.
Clear as mud?