Bolt-head differences (AR-15/M16)

Also keep in mind that many AR-15s come with M16 bolt carriers (often referred to as full-auto bolt carriers). The different types of bolt carriers are completely interchangeable, the only thing is that a semi-auto bolt carrier won’t trip the auto-sear on an M16/M4.

Paranoia about bolt carriers has run in cycles. Originally, to get approval for the semiautomatic AR-15, Colt cut back the rear of the M16 bolt carrier so it wouldn't trip an auto sear. Later, it introduced the "beveled" or "unshrouded" carrier as part of its system to prevent hammer follow-down. Still later, it made a carrier with the entire rear open at the bottom, to prevent an adapter from being added to trip a DIAS.

ATF, at one time, ruled that a full-auto bolt carrier was one of a list of "no-no" parts that were not supposed to be found on a semiauto gun (the others included the selector, hammer, trigger, and disconnector). Colt apparently built up an excess inventory of full-auto carriers and it finally got ATF to relent and allow them to be used on semiauto guns. These have since become the industry standard.

The barrel extension and bolt set the chambers headspace. This headspace should be checked with a gauge.

Different bolt= different headspace.