The handgun owned by Mr. Stanley was a WWII era Soviet Tokarev T-33 semi-auto pistol that fired a 7.62x25mm cartridge. Using Soviet milsurp ammo, the average muzzle velocity expected would be just over 1300 fps. The milsurp ammo used by Mr. Stanley was actually made in Czechoslovakia in 1953 for the CZ 52 semi-auto pistol; the Czech version of the TT-33.
The Czechs changed several features when they designed the CZ 52, including replacing the simple locking system that kept the action in place, preventing the rearward travel of the action until the bullet had safely exited the barrel. Because of these changes the Czechs must have felt their CZ 52 was capable of withstanding higher pressure chambers than the TT-33, and the loading of their ammo reflects this.
The article linked to below was written by a Mr. Claude Harrington, an expert on the Czech CZ 52.
https://harringtonproducts.com/7.62x25mm/
Take note of the second paragraph, in which Mr. Harrington states that Czech ammo is loaded up to 25% hotter than other Tokarev ammo. While he does not come right out and warn against using Czech ammo in anything but the CZ 52, he does caution against using it in guns not designed for it. I believe this warning would also apply to 75 year old TT-33 pistols that were dirty, stored outdoors and likely corroded, worn and possibly defective.
Also interesting in this article is the ballistics table near the bottom, showing the Czech ammo clocked at 1640 fps, while another 85 gr. JHP was only clocked at 1230 fps. Energy at 1640 fps was 512 ft/lbs., while energy at 1230 fps was a mere 290 ft/lbs.; almost doubling the energy.
As ammo ages, the primers and gunpowder that make up its propulsion system will deteriorate, and are typically the cause of hangfires and misfires. This was especially true of a lot of older European milsurp ammo, as it was common to use corrosive compounds such as sodium or potassium chlorate as oxidizers in their primers. However, it is not always true that deteriorated gunpowder automatically loses its potency. In fact, it has been found, in many cases, that aged gunpowder, through chemical transformation, will burn hotter than when it was new.
Now, back to the question; how did the casing end up bulged out at the base, and its bottleneck swollen out to the diameter of the rest of the slightly tapered casing?
To understand my explanation, I must first insure you are cognizant of exactly how a short recoil blowback semi-auto works, and just how delicately balanced the operation is.
The whole point of designing a short recoil blowback (SRB) system is to time the departure of the bullet from the barrel with the rearward travel of the action (and extraction of the casing) so precisely, two events are avoided:
1. The casing is not started moving rearward out of the chamber before the bullet leaves the barrel. Such an event would allow contained high pressure gases in the barrel to blow back into the shooter's face.
2. The casing is not started moving out of the chamber so late, the expanding gases necessary to move casing and action rearward have all escaped out the muzzle with the muzzle flash.
In other words, what appears to be a very simple reloading system is actually a complex balance of mass, spring tension and rate of powder burn that must be timed to within a mere fraction of a second just to get it to work at all. Change ANY of the factors involved, and the results are immediate and obvious.
We already know Mr. Stanley had a TT-33 that was possibly up to 80 years old (first mf'd in 1934) and that he was shooting Czech ammo that was loaded up to 25% hotter than the pistol was designed to shoot. We also know he stored this ammo outdoors in a shed in a climate that varied from -30° to +30° C. We also now, from testimony, that Mr. Stanley did not actually shoot coyotes with his TT-33 but, rather, employed it more as a noise maker, due to its exceptionally loud muzzle blast. Therefore, it is doubtful Mr. Stanley ever cleaned the barrel or action on his TT-33 or paid much attention at all to its maintenance. We also know the TT-33 was kept outdoors in the same shed as the ammo, and there is a very good chance its action and barrel were rusted and possibly corroded.
https://upload.wikimedia.org/wikiped..._operation.png
Above is a link to a diagram showing the five stages of a typical short recoil operation cycle. Below is the text describing each stage.
Cycle diagram explanation
Block diagram of short recoil operation cycle.
1. Ready to fire position. Bolt is locked to barrel, both are fully forward.
2. Upon firing, bolt and barrel recoil backwards a short distance while locked together. Near the end of the barrel travel, the bolt and barrel unlock.
3. The barrel stops, but the unlocked bolt continues to move to the rear, ejecting the empty shell and compressing the recoil spring.
4. The bolt returns forward under spring force, loading a new round into the barrel.
5. Bolt locks into barrel, and forces barrel to return to battery.
As you may have guessed by now, rate of powder burn and peak chamber pressures are critical factors in getting the bolt to unlock from the barrel at PRECISELY the right moment.
I believe the 3rd round in Mr. Stanley's TT-33 was not only a hot Czech load, its powder had also experienced chemical changes over the last 60+ years that caused it to burn at a higher than normal rate. The resulting elevated chamber pressures first caused the casing to expand, fire-forming it to the precise bottleneck shape of the chamber. At this point, though, due to extreme chamber pressures, the barrel and bolt began moving rearward together faster and with more force than usual. At this point, the worn/rusted/unoiled/corroded locking lugs disengaged the bolt from the barrel prematurely and, just before the bullet exited the barrel, began moving the bolt and casing rearward, just out of the chamber. As chamber pressures were still elevated, the already fire-formed casing now re-expanded to the shape seen in the photo. This re-expansion of the casing only stopped when a) the bullet left the barrel, relieving gas pressure or b) the slightly tapered 7.62x25mm casing moved far enough back in the chamber to release gas pressure rearward.