trueg50:
At Murray's Gunsmithing, in Bowie TX (which worked on my chamber), Mr. Murray's best conclusion about the popped primer cause seems to focus on different amounts of freespace in chambers. Tula ammo is not the only type experiencing this. The contours of my Yugo firing pins are identical to other originals: nothing was modified.
To clarify, Mr. Murray suspects that during production in various countries, chambers were manufactured with different amounts of freespace.
This can be so tiny in some guns that excess pressure in the case is caused when the bullet has almost no room for forward movement, if I understood his replies to my questions correctly at SKSboards.
His conclusions about "freespace" (being the primary cause) result from the fact that After such precise chamber reaming, no more popped primers have been seen in the many rifles which were modified.
The dimensions of the popped primers' holes are the exact dimensions of the firing pins' holes, caused by the rearward gas flow due to such excess pressure in the case.