Complex sentence: The bolt carrier and internal lower receiver of semi-automatic versions are milled differently, so that the firing mechanisms are not interchangeable, although the full automatic M16 bolt carrier is now the most popular type, and approved by ATF.
Split into simpler sentences: The bolt carrier and internal lower receiver of semi-automatic versions are milled differently, so that the firing mechanisms are not interchangeable. Even so, the full automatic M16 bolt carrier is now the most popular type, and is approved by ATF.