Updated 11/27/2008DISCLAIMER: I've only been exploring this topic for a few weeks. I've used the extension tubes in a few matches and they seem to work good. However, there are important issues that need to be paid careful attention to as mentioned in the processes below. I am not a gunsmith, just a dude with a gun, camera, and dremel. Your mileage will vary. I do not use these magazines for self-defense, only competition
Well, here's the semi-step-by-step on how to install the Pearce +2 Beretta magazine extension basepads.
New follower on the left, modified on the right. Note the shorter height, and rounded edges. WARNING: ensure that the follower is still tall enough so that when the pistol runs dry, the follower isn't pushed out the front of the magazine feed lips and into the feed ramp of the barrel.
New follower on the left, modified on the right. Note the shorter overall front-to-back length.
New follower on the left, modified on the right. Note the rounded edges again.
New follower on the left, modified on the right. Note how the rounded edges sweep under the follower. WARNING: ensure that the follower is still tall enough so that when the pistol runs dry, the follower isn't pushed out the front of the magazine feed lips and into the feed ramp of the barrel.
These are some measurements taken with a dial caliper. What works for me, may not work for you. However, these are the dimensions of one that is match-proven to work.
Begin with a dremel with a miniature cut-off wheel to trim the height down.
Use a drum sanding bit with the dremel to round the edges and trim the piece down to size. WARNING: removing too much off of the top of the rear edge may affect reliability to lock the slide back after running out of ammo.
Now the new follower is starting to look like the modified one.
One of the ups and downs of the extension is the huge hole in the bottom of the pad. this provides no retention to keep the extension centered under the magazine tube. However, note that when the magazine is fully loaded, it won't be going anywhere, and can easily withstand a reload. when an empty or near-empty magazine hits concrete, the extension wil slide off a little, but stays in place due to the spring. The large hole also helps in tuning the follower, which I'll show you next...
This is the magazine tube upside-down, with 13 rounds of 40sw loaded up. The follower is sticking out past the bottom of the tube, and now you can see why you need to round the corners. The inside of the extension is actually a hair smaller than the tube. It makes the follower going in a bit difficult, but it completely ensures upward travel as rounds are expended from the top of the magazine.
In order for the follower to work reliably up to 15 rounds, you not only need to round the follower edges - but you need to horten the length from front to back. Here, the follower shown still needs to be shortened. The best test to know whether or not enough material has been taken off: place the follower as shown in the picture. Shake the extension sideways. The follower should jiggle freely inside the extension.
Why won't 16 rounds fit? Well, when there's no spring in the magazine, 16 rounds fit fine. But with the spring in place, this is the most room you can get above the 15th round - about 4 millimeters. Unfortunately, the magazine spring is completely compressed at this point.
Two extensions cost me about 24 bucks shipped. I ordered four more for 50 bucks. Fifteen+1 isn't ideal for Limited division USPSA shooting, but it's better than 12+1, or buying a Tactical Sport at this point.
In addition, don't throw away your old basepads. If you ever need to load with less than 14 in the magazine, the extension will slide out of alignment when you slam the magazine in for a reload. So, if you need to shoot Limited-10, swap the basepads back in. I haven't found a good way to solidly lock the extension in place. I have some ideas though, so stay tuned...
Hope this helps answer some questions out there!