AR15 M4 M16 Headspace:
There seems to be quite a bit of confusion relating to the headspace of an AR15. There's no point of me trying to explain it when others have done it more eloquently then I could ever hope too. Below you will find links and references to help you better understand the headspacing of an AR15.
Safety Note: Whether your building your own AR15 or buying a used rifle you should always at the minimum before a headspace check with a field reject HEADSPACE GAGE. There is a common misconception that when buying a barrel with the barrel extension attached that because it is Mil Spec that it will automatically be correct. Do not assume this to be true. Verify the headspace with a FIELD REJECT HEADSPACE GAGE or take it to a gunsmith and have it checked out.
When ordering a barrel its always best to provide the barrel supplier with your bolt so it can be properly headspace in advance. The bolt should be kept with the barrel. Meaning: If you swap upper receivers each upper should have a dedicated bolt. It ok to use one carrier but do not swap bolts!!!
Note: GAGE is misspelled GAUGE almost everywhere. Both terms may be required when searching for information.
Be certain to choose the correct caliber 5.56 or .223.
See below for specific selections