Just a thought - appears to me that:

1/. the shoulders are filling out well, but perhaps the necks are a bit hard and not sealing due to the low ballistics you are producing. I assumed they should be running in the 2,300fps range.

2/At 2,000fps they are sub or low-pressure loads and the powder is still not burning well. I seriously doubt any of the 4831's are suitable for lower pressure NE - type loads.

They were designed for ctgs. that run in the 60,000psi to 65,000PSI range.

www.kwk.us/pressures.html lists the Gibbs as a 39PSI CIP Pezio range (35,000CIP crusher. These pressures are not conducive to good, consistent ballistics with THOSE powders.

Since your pressures MUST be considerably lower than that due to the velocities you recorded, THAT is most likely your problem.

I have no experience with the .505, nor any other NE round - but - your brass is showing all the signs of too-low pressure - OR - too hard in the necks to seal - as well - both probably.

On the other hand, as well, this can happen if during a hangfire, the bullet is ejected into the throat ahead of the neck by a minor parcical ignition, and pressure stops building, then the powder's ignition re-manifests itself, developes normally and spits it out the tube - nasty hangfire is evident. The sharp shoulder had sealed, but the necks didn't seal before powder gases got between the neck and the chamber neck - collapsing it as it increased.

I would research, then do some more loading - with faster burning powders. Neither of the 4831's nor 4350's are suitable for such low pressures.