Recoil Springs for converted Hi Power 40S&W to 9mm

General Firearms Handguns - Semi Auto
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So I had MK3 firearms build me a sweet Browning Hi Power, but what we did was take a 40S&W and convert it to 9mm.

So my question is as follows the tradition recoil spring weight for a 9mm Hi Power is 17 pounds.

But in the 40S&W converted to 9mm we have some changes that I think need to be taken into account.

  1. The 40S&W has 3 locking lugs and not 2 like the regular 9mm from the factory Hi Power.
  2. The slide of the 40S&W weighs 3 oz more than the original 9mm slide.

My question is can I get away with running a 15 pound recoil spring in the pistol due to the difference in the slides?

The slide travels smoother with the lighter spring.

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Our pistol smith would ask “what ammo do you wish to shoot?” Hot carry? Factory 124s? low power factor competition?

15 lbs sounds like a good starting point if you’re not running soft loads.

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Wolff has variety packs so see which gives you reliability without launching brass in to the next county.

Stephen Camp has been dead for several years and unfortunately the Hi Power forum went with him which would have given you access to several with first hand experience.

You might be able to dig around in this and find a direction to go.

Not sure if the .40 pistols used the same hammer spring as the 9mm, but that could be a variable and/or something to get some adjustment with also.

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MK3 had put a 17 pound recoil spring in it.

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.

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Is that .40 frame vs forged 9mm, cast 9mm, or are all the Mk III frames the same weight?

I remember reading the .40 traditional forged frame wouldn’t hold up to the cartridge(although i can’t recall if that was pre production R&D or testing by customers) and the fix was a cast frame and soon after the 9mm frames became castings also due to being stronger.

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Got to thinking, you might ask for/look for recoil spring tuning the FM90 copies from Argentina.

I assume the slides would be closer in weight to your .40 slide since FM didn’t do the slide scallop cuts.

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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.