But that is the stock weight for a Hi Power 9mm and I think that in a 40S&W converted to 9mm with the extra weight of the slide and going from 2 locking lugs to 3 the round will use a lot more energy to move all that mass and resistance over a stock 9mm slide.
Besides I am tired of Iran talk. This is so much more interesting to me.
I have not shot the pistol yet, because the KKM barrel was fitted to the slide and frame by MK3, but I wanted it black isonited so it will look better in my black finished pistol. But KKM told me that they see a 100 to 150 fps increase in a regular KKM barrel and a finished one due to the reduction in friction. So the 9mm conversion barrel is back at KKM for a 2 to 3 week period.
Some fun facts MK3 milled in a high undercut and added a light rail. I would have thought the 40S&W frame would be heavier than the 9mm but they weight the same. Like I said the slide of the 40S&W is 14.9 oz and the 9mm slide is 12 oz .
Total weight for 40S&W is 31.04 and the 9mm is 28 oz , a difference of 3.4 in the slide.
When I did these conversions. I always used the original spring and work down if i had too. Mine was not a MARKiii, it was early 2000’s so who knows. The OEM 40 spring worked flawless with my weak hand loads.
I had more issues with the extractor than I did with the springs.
You will notice if the slide is slamming into the frame with a too light spring and feel it in the recoil impulse. BHP Spring Solutions had the spring kits for rebuilding my high power finds and they have a lot of material to review. Maybe they have experience with this conversation and can add some pointers.
Typically small recoil spring weight changes are done to help the sights return to point of aim when firing a fast pair. Too strong and muzzle will dip when returning to battery. If OEM is 17 lbs, 15 sounds very reasonable as long as reliability is maintained. I run my M&Ps, 4” and 5” with 15 lb recoil springs w/o issues.
