Bullet Run-Out and Case Mouth Concentricity Problems

I recently had some trouble reloading my .416 Rigby. I have gotten into the good practice of running all my brass through the action of the rifle after I've resized (I also perform full length resizing), trimmed, deburred, clean the primer pockets, and tumbled them. During this particular reloading session, the brass cycled perfectly. I then completed the loads (primer, powder, and bullet) and ran them through the action again(one must be excessively careful here...these are live rounds). Not a single round would chamber! I have been reloading since the 1980's and have never witnessed this problem.

So I began to research the problem and test various solutions but remained stumped for much of the day. Why will the action not close on this rifle?

I approached the problem deductively...first, we know the brass was OK before loading the charge. Secondly, the seating of the primer and addition of powder can have not bearing on the problem so I ruled out that step in the process. No...it must be something with the bullet seating.
Headspace I thought. If this were indeed the problem the brass should not have chambered either, but just to be safe I pulled some factory ammo and chambered a round or two. No problem at all!

Maybe the bullet seating was inadvertently applying a crimp which was causing a slight case mouth bulge or non-concentricity. To test this, I only partially seated a few bullets in properly sized brass. The problem continued to plague me!

Then I stumbled upon something on the web regarding bullet "run out". This is a condition where the bullet is not seated straight in the case mouth and subsequently bulges the neck of the case and/or changes the shape of the shoulder. Recall that this is a .416 Rigby and it is the shoulder that is responsible for proper seating in the chamber...and it is the shoulder that is responsible for determining headspace. This is not the case with the belted magnums. The primary cause of bullet run out is an insufficient chamfering of the case mouth. Chamfering is done during the deburring process. It seems so trivial that I had little faith it would solve the problem, but I tried it anyway. I chamferred and deburred the case mouths very well and then placed in a bullet. The result was fantastic and a valuable lesson was learned.

To learn more about this condition, click to watch a short animation. (note: applying any of these suggestions is done completely at your own risk)