Nothing here is an attempt to change the way the pistol operates. All work was done with 400 and 1000 grit W/D sandpaper and a buffing pad. Tools required for disassembly include needle nose pliers and a couple small punches.
Don't remove any metal you don't have to. The sandpaper is available at most auto parts stores and the buffing pad is available at Dremel retailers.
Disassembly of the USP is not documented here. Detailed instructions can be found at USP Disassembly.
Use your favorite gun lube/grease when reassembling.
Follow the disassembly instructions and you will end up with a pile of parts that looks something like this:
Note: The flat spring and the shaped spring (slide release) have not been removed. They would be the last parts out and the first parts in, so there is no point.
Smooth the trigger bearing surfaces as shown:
Smoothing is best done with the sandpaper on a hard flat surface. The main surface you should worry about is the one that contacts the trigger bar. Then polish with the buffing wheel chucked up in a drill press. Add a small amount of your favorite polishing compound to the wheel for each item. Buff, till you get a smooth reflective shine.
Insert the trigger and push it forward. Insert the trigger axle just enough to hold one side of the trigger in and then with the needle nose pliers insert the spring (you may need to twist along the long axis of the frame) and push the pin in. If the pin doesn't line up, push on the trigger to line up holes in the far side and push the pin in.
The trigger bar hooks to the trigger and operates the hammer in double action. These surfaces are key to reducing the double action pull weight and improving the feel.
The back of trigger bar touches the disconnector (top rear), the hammer (the hook) and the trigger bar detent (the bottom). All of these surfaces need to be smooth. Also, I polished the top of the trigger bar detent.
The hammer strut pushes the hammer with the force of the hammer spring. I smooth all four sides of the strut where the hammer spring rides and the top of the strut where it contacts the hammer.
After smoothing the hammer strut, insert the trigger bar detent (use the needle nose pliers) into the frame. Drop the hammer strut in and then insert the trigger bar. It should look like the following pictures.
The bottom surfaces of the hammer contact the trigger bar when shooting double action. These surfaces need to be smooth. Make sure that you don't round off any of the edges. One of these edges contacts the sear, the other the trigger bar when shooting double action. Rounding them off will change the trigger pull for the worse and may make the pistol unreliable. I smoothed the sides of the hammer, but I'm not sure it is necessary. If you do it, do not go too far up the side of the hammer or you will see the bare steel when it is assembled. The hammer axle is just a rolling bearing surface for the hammer. Smoother, the better.
Drop the hammer into the frame and insert the hammer axle. Make sure that the hammer strut is in the slot on the bottom of the hammer. If you cannot pull the hammer back, it is probably misaligned. It should look like this when done:
The detent slide pushes on the detent plate. Smooth it too. It will make operating the control lever much easier.
Insert the detent slide (with the spring on it) into the frame.
Next, insert the control lever. You have to push the detent slide down, compressing the spring, when you insert the control lever. I do it with a small punch.
Smooth the parts as shown. These parts contact things like the firing pin block, the top of the trigger bar, etc. I also polished the sides since they rub against each other.
Reinsert these into the frame and insert the sear axle. I find it easiest to use a small punch inserted from the left side of the frame and the pin inserted from the right side of the frame. Gently move the punch in a circular motion to line the parts of and push the pin in incrementally as the parts line up.
With the pin inserted, it should look like this:
I smoothed the inside of the detent plate which helped the feel of the control lever.
I use a punch to hold and align the detent plate in and push sear axle in the rest of the way from the far right side:
Note: It may be necessary to rotate the control lever slightly to insert the detent
Check the hammer strut again to make sure it is in the slot on the bottom of the hammer and slide the hammer spring over the strut. Make sure the hammer is down or you will have to compress the hammer spring more. Insert the lanyard loop insert into the bottom of the frame and align the hammer strut and spring. When aligned, insert the lanyard loop insert pin. This completes reassembly of the frame. Ensure that the trigger can be pulled and that everything works correctly before putting the slide back on.
I have done this trigger smoothing procedure to all of my USP (compact and full size) and there is a noticeable improvement including the "Match" version. I can only speculate that it will also help the LEM version. It is possible to install the HK "Match Trigger" into a USP compact. All parts are used except for the trigger itself. You can use the trigger stop in your compact trigger by drilling and tapping the hole. It is only plastic, so care must be given to procedure. For 30 minutes of your spare time, you'll have a much better feeling trigger. Stay tuned for the modified trigger pull and shorter reset page.