Anyway, this is one I did awhile ago on my old police trade USP .45 FS. Same mod as for the USPc.
At left is the original USP catch. In the middle is an unfitted HK45c catch. On the right is the fitted catch before the paddle reduction.
Basically, the raised bar between the paddles needs to be shaved down to be flush with the rest of the forward-facing surface, so it fits like the USP catch. It might be half a mm of material to remove. Pretty simple. Shave with an X-Acto knife, file it flush and sand to make it smooth and clean up the edges. On the end of the tab itself (the part that engages the magazine notch), I also knock the peaks of the roughness off the stamping fracture surface and remove the burr.
With a mag inserted, the area between the paddles that I've shaved down is raised away from the trigger guard at least enough for a thick piece of paper or business card stock to slip freely between them, so I know the position of the mag catch is being controlled by full engagement with the mag.
Attachment 42557
The problem with not fitting it is that the raised bar runs into the relief on the underside of the trigger guard and prevents the catch from rotating all the way upward. This prevents the tab from protruding far enough into the magwell. It will engage the notch on the magazine, but not fully. Rather than being fully engaged in the notch, it's just hanging on the edge of the notch. There are reports of guns dropping mags under recoil. It's not clear to me whether that occurs immediately or if the problem is that over time, the tab wears the edge of the notch and softens/rounds the edge until the mag is no longer securely held. Possibly both. (The answer to most multi-factor problems is "it depends.")
The wear/damage issue is especially of concern with the plastic bodies on the full-size USP 9/40 mags. I haven't yet done this on a full-size USP 9/40, so I can't say for sure it will work properly there, but I would apply the same evaluation criteria. I do expect that the smaller, steel tab would be fine if it is fully engaged in the molded pocket in the magazine. I think the problems result from it engaging only the edge of the pocket. I do intend to test that with a few tens of thousands of rounds through one of these guns eventually. Just not there yet.
You also need to check the paddles for interference with any holster the gun will be used in. There's no good reason to assume that holster makers provide room for the larger paddle in their molds, and interaction with the holster could conceivably cause a mag to become disengaged from the catch and not be noticed until some time that it's really needed.
It's really, really important to get all the details of this right, because dropping mags under recoil or at another unintended time can get you kilt in da streetz.
I've posted my observations and reasoning here. If someone can explain how I'm wrong, I'd appreciate being told about it. I believe I've gotten it right, but
I am not a licensed gunsmith or gunsmith instructor, so any action you take is entirely at your own risk.
Ergonomically, the unfitted catch also causes the paddles to sit a little low, underhanging the trigger guard, which makes their oversizeness even more annoying. So fitting it puts them in the correct alignment, then the reduction makes them the right size and shape, which is entirely to taste.
ETA:
Thinking about it further, it's conceivable that some shooters might inadvertently activate the oversized paddles due to gun movement during recoil and simply not notice that's what's happening. Which would be another reason to pay attention to the paddle reduction aspect. They only need to be big enough to work well for you, not any bigger.
Originally Posted by GJM
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