[P Flados]

You are not the first. I have seen similar photos from Winchester factory 350 L ammo. Some have simply been where the brass takes on the look of a belted round, more extreme are the blow outs and case splits.

It is pretty obvious that a gross overcharge of AA1680 can not be be the cause. The powder fills the case with normal loads. I have loaded some compressed loads with WC 680 (milsurp equivalent) in my 357AR. Pressures were too high for my liking but nowhere near the pressure needed to flow the base.

Your AA 1680 load is also very unlikely to have partial ignition followed by a spike. To get your velocities, you have to be above 90% load density. High load density is not generally associated with a secondary pressure spike.

Larger than bore bullets (by 0.003 or so) are used all of the time. The resistance of the bullet being squeezed down can increase pressure some but this should be predictable and consistent. Having a bullet get cocked, jam or otherwise hang up is very dependent on bullet shape and "freespace" in a chamber. In no way do I see the profile of your Speer getting hung up in your chamber.

Just for the sake of argument, can you estimate either the free space above the powder or the amount of compression for your loads in your brass.

Now lets discuss those other failures. They were attributed to one or more of the following: Over-length cases, over crimped brass, bad chambers.

The 350L does headspace on the mouth. On the drawing, there is a distinct step at the front of the chamber. The dimension shown is 1.710 - 1.720 in front of the bolt face. The case mouth is supposed to be resting against this step when the round is fired.

If a round has too much crimp, is too long, or if the chamber does not have the correct step at the front of the chamber, a small amount of case can end up just forward of the step. In this configuration, you have a crimp that will not release. The bullet can squeeze out, but the pressure gets way over normal due to the extra resistance to initial bullet movement. The case mouth may look close to normal afterward as it expands after the case slides rearward in the chamber during the overpressure spike.

I do not know what caused your event. It is really hard to see it as a powder charge problem or an oversized bullet problem. A way out there possibility would be an ignition problem. As best I can tell, all other possibilities involve the end of the case getting in just past the headspace step in the chamber.

Edit: I wrote the above before seeing the post on the second "problem" round with starline brass. The 350L should have the transition from case wall to web occur inside the chamber. Too much rounding at the chamber entry, too much headspace, or "bad brass" could result in some unsupported case wall. The amount of extrusion of the blow out round does not seem to match with a failure due to unsupported case wall. Run a 5/16 rod, squared off pencil or other item into your brass an see how far down the wall goes. This will tell you where the case wall to web transition is.

For the failed round, the extrusion on the face into the hole for the extractor pin is typical for a pressure spike, this would not be there for a simple unsupported case wall failure.