i wouldnt go so far as to say ar barrels are self headspacing, cuz it aint so. now because of modern machining, your odds of putting one together and getting correct headspace is pretty good, but not guaranteed. unless of course the pre-chambered barrel you purchased also comes with matching bolt. without a set of guages, you can not guarantee your headspace is in spec. never assume anything.
The Sierra Match King bullet used in the Federal GMM load, and many other factory match loads, does tend to fragment on impact with the jacket separating from the lead core and then the lead core tends to penetrate fairly deep. This needs to be kept in mind when planning your shot and insure there is a suitable backstop behind the target. I ordered an excellent norinco sks barrel from e-gun parts and went to install it. Since there all matching numbered and orginal i deceided to leave them alone.