My understanding is the recoil spring has very little to do with the timing of the slide and barrel unlocking. The bullet drags the barrel forward as it leaves the barrel and the locking lugs determine when the breech unlocks from the barrel. Slow motion video clearly shows bullets leaving the barrel before slide movement. Spring rates do more to determine how fast the slide goes forward; my experience is the rearward timing does not change much with full power loads. Full power loads just slam the slide back regardless of spring rates. I think the G20 comes with a 17lb spring, I have an STI 10mm that has a 24lb spring, a 10mm Kimber with a 18lb spring, and a DW 10mm commander that I think has an 18lb spring as well. All function, all different spring weights. And yes, different guns.
My understanding of the polygonal rifling is that it lacks the sharp edges of traditional lands and grooves and does not grab soft lead bullets well. So the bullets sort of get mushed forward leading the barrel and not spinning properly. Hard cast lead reportedly does not have that problem.
I understand the logic of soft lead bullets as you have described, but in describing cast bullets I am presuming hard cast bullets. Maybe I’m off base in making this assumption? In a broad statement attributed to Glock technicians that recommends no cast bullets I would certainly presume that includes hard cast bullets. If this is so, the question of why this is a problem remains unanswered.
