http://www.handloadersbench.com/view_post.php?post_id=265433 The 223/5.56 has a military standard bullet pull of 40 to 45 lbs . Measure the neck before and after seating the bullet. Seating of the bullet should expand the neck .002" to obtain 40 pound of bullet pull or the correct minimum neck tension for an AR 15 or M16 rifle. The shank of the bullet must come in contact with the case neck. When using a long COL to reach the rifling, this will reduce neck tension. Very low or light tension may not let slow burn rate powder work correctly. Squibs may be the results in large magnum cartridges.