Here's how the arm is supposed to function on a GS.
Now, the Wilson drop-in may drop right in, or it may not. If it doesn't, then you'll need to find out why. It will be blocked from dropping in and functioning at one or both of two areas: at the very front, tip of the GS arm, or along the bottom horizontal flat that will ride above the arm when the safety is disengaged (gripped).
Now, before you get ready to reinstall the GS to try it again, make sure you have a good, strong light and also, preferably, some magnifying goggles. Leave the thumb safety out of the gun. Most folks eventually figure out that they can look up inside the mag well and watch the GS arm work against the trigger bow, but they may not know that you can also see a bit through the GS hole.
Once you're ready to go, start by slicking up the trigger bow. I use very fine stones and paper. Don?t change any dimensions. Just get everything smooth and the sharp edges broken.
Next, grab the ol' Dremel with a wire wheel brush and remove all bluing or park (or whatever they come with) until I have nothing but shiny metal at the tip of the arm and along its lower flat. Then, color the now shiny area with a magic marker.
Next, carefully (so as not to disturb the coloring, try to put the GS in place in the tangs. Look up through the mag well and through the TS hole and see where it seems to be hitting. Then pull the GS back out and look at the marks to see if they confirm what you thought you saw.
I remember that the first Wilson GS drop-in I got came with no instructions, and the second one came with what I consider incomplete instructions. With regard to the latter, the instructions said that some material might need to be removed from the bottom flat. What they didn?t say was that the very front tip of the GS arm might bind, and that if that happened, that would have to be relieved even before getting to the lower flat. The following graphic shows what I'm talking about.
See the point at the tip of the arm? If that contacts the wall, the GS won't go in. You'll have to dress the tip back by removing VERY TINY amounts of metal until the GS will install (meaning until the thumb safety will install in the gun with the grip safety in place).
Now, once you get that done, then you?ll need to be sure the GS operates ? that it blocks the rearward travel of the trigger when engaged (the GS's bottom pad is all the way back) and that it will free the trigger to travel when the GS is disengaged (when the pistol is gripped).
To do that, smoke things up again with the marker, look and test as above. If you need to remove metal from the lower flats, the following graphic shows where.
Go slow, slow, SLOW! If you take even a micron too much metal off, the GS will no longer engage!
Once you have things right, then I like to take one last step. With the gun completely together and after making sure it is unloaded, the muzzle pointed in a safe direction, etc., do this in this order...
1. Cock the pistol and dry fire it, and hold the trigger back (don't release it).
2. Grab the slide in your other hand and, while still holding the trigger back, open your grip your shooting hand has on the pistol. What you're trying to do is completely release your grip on the grip safety while still holding the trigger back in the "bang" position.
3. Slowly release the trigger.
Did the trigger bind? In other words, did it spring all the way back forward on its own? Or did it seem to catch on the GS's bottom flat? If it caught, the trigger will either not spring forward all on its own, or it will feel gritty. If either of those is the case, take the GS and trigger back out. Verify that you have completely smoothed the back right of the trigger bow, including the top corner. Then, with extremely fine stone or paper wrapped on a file, just polish the bottom flat of the GS until it shines. DO NOT REMOVE ANY MORE METAL - JUST POLISH IT UNTIL IT'S GREASY SLICK! Then, reassemble, and repeat 1-3 above. The trigger should no pop right back out with no binding gritty feel.
This is an extra step that most folks don't take. They argue that it doesn't matter what whether the GS and trigger bind when the gun is operated like 1-3 because when you release the trigger, you're always holding the GS depressed in real-life. That's probably true, but I was taught by my first instructor, Dave Sample, to "finish the job," and to me, that means that I want my job to be the best that I can do it.