Macay, Buffalo Bore recommended heavier springs in the Glock 20 in 10mm with their 220-grain cast loads and they also said shot-to-shot velocity variation went way down. Just as you experienced. My accuracy improved with the heavier spring and 220-grain cast loads.
This quote from BB that SU35 posted explains it better than I can
https://www.24hourcampfire.com/ubbt...glock-10mm-with-heavy-loads#Post16891543What the owner of Buffalo Bore experienced in his testing is pretty spot on and parallels what I have done and observed. I generally don't take anyone's word for things unless I know them personally or know their background/credentials and testing processes/procedures. That said, what the owner of BB has found in his results and loads is right in line with my own.
Your observation that accuracy also improved with the heavier spring/heavier projectile is one that I have noted as well.
I have been doing a substantial amount of testing of a heavy .40 load as of late. Using a polymer coated 170 grain SWC and have gotten some truly excellent accuracy. Traditionally .40 Glock barrels have not been noted for their stellar accuracy, but in testing I have noted that these heavy loads shoot very very well.
I will likely offer a heavy 170 grain load specifically for 3rd Gen and newer Glocks, but I may actually sell it with a guide rod and heavier spring as an added measure of safety to ensure that people are properly set up, since I have seen far too many Glocks with completely worn out RSAs, and people assume that if the gun is still functioning then it must be OK.
