Thanks everyone for your kind comments. Yes that shot on Wednesday definitely got my attention. I've reloaded shotshells for over two decades now and never had anything like this happen. Here's some additional data.

The charge of Alliant E3 I use is relatively light and this morning just for test I loaded one Gun Club hull with 1.5 X that powder charge weight and the rest of the loading operations went as usual. The crimp looked typical and without a hint of a bulge. Then when I cut that shell apart I found the powder tightly compressed. Those of you who load light so-called "low pressure" shotgun loads with any powder should be aware of this. Incidentally that 1.5 X weight of Alliant E3 is way over the max shown in Alliant's loading data for my GC hull and components recipe.

One other possible factor: like most of my friends I use a "baffle" beneath the powder bottle to decrease the range of variability of the powder drops. Now, I don't know if it had anything to do with clumping of powder I left ~6 weeks in the press. You'll see in the last pic (looking down from the powder bottle) how there are two baffles that each block about 1/2 of the cross-section and cause the powder flow to reverse before getting to the powder bushing in the charge bar.

Frank that is very interesting that a 1.5 charge did not show externally. The wad has enough room to allow that with out causing a bulge. Thank you

Contrary to what some believe a powder baffle is good for, it can impede powder flow. Especially powder that has had 6 weeks to absorb moisture. You will note that MEC does not make or offer powder baffles for its machines. Only certain aftermarket makers.

John is right on about those powder baffles. I bought one many years ago when they first came out, and after testing with several different powders I found it made the charge weight much more inconsistent with some powders. Never used it again. Also it is best not to leave powder to long in the loader, It is both light and moisture sensitive. Here in the Pacific NW moisture is always a problem and I have had it clump up surprisingly quickly.

I had an opposite experience and found the baffle more consistent. When I bought it some years ago I tested the powder drop weights (with/without) while using IMR "PB" powder and a particular bushing I was routinely using at the time. The extreme range of variability (3rd sigma as in statistics) was > half less with the baffle, than without. That's with 20 powder drops in each mode and the data is in my reloading log book. I still have a lot of "PB" squirreled away and like it for composite barrels but have gone to 700-X for paper shells and to E3 in plastics for my casual 12-gauge clays shooting with steel barreled guns. Just the way I do it …….

No, I didn't do the with/without drop weight testing using the baffle with either of the latter two powders.

Those baffles are junk, I remember having a problem with light drops and threw the one I had away in the 1980's when I was loading for ATA shoots on a MEC 650.

I have had trouble with the spring return of the charge bar on my progressive mecs. Usually it was a piece of shot that got sheared off and would stop the bar from going all the way back causing a lite charge of powder. But anyway, the baffle could not have caused the 1.5x charge because it is above the charge bar and bushing. The bridging would have to have been in the drop tube somehow.

Thanks for the heads up Frank. Glad you and the Winny came through it with no damage. I've been using that exact baffle with no problems yet and it sits atop one of the multi-charge bars which some people love to hate. I not sure what went wrong with your reload but I think dumping the powder after each session is wise. Could static electricity had anything to do with it?

Great story and investigation Frank. I shoot a lot of trap at big shoots so shoot with lots of different shooters. Can think of many instances of squib loads and wonder how many overloads go along with these? Scary stuff for sure.

Thankfully this experience happened with a M 21. I have reloaded for quite a few years, and have gotten into the habit of visually inspecting shells as I load the gun.
An abnormal looking shell goes in a pants pocket for later review. Usual issues are a tipped wad.