I haven't seen much of any talk about these, so figured I'd share some photos and other notes and make what is probably the most helpful post I've made on this forum.
I'm not sure if Apex is still producing these or not, but I know they are available through a couple online retailers (I got mine from Brownells). Apex does not list them on their website or their YouTube channel.
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In the package you get the +3 extension, set screw, and allen wrench. The Apex Tactical card refers you to YouTube for install instructions. They won't be there, don't bother trying, but it's nothing hard.
To install, unload your magazine, flip it over and using a small screwdriver or the like, depress the lil button on the magazine plate of your magazine and slide it forward. Try to remember to keep control of things as there is still spring tension on the bottom.
You'll want to remove both the magazine plate and the magazine plate catch (from inside) and put aside for storage if you ever want to un-do this. Don't leave the plate catch on the end of the magazine spring or you'll only end up with a +2 extension.
This is a good time to clean up your magazine internals if you haven't done that. No lube, just a wipedown is fine.
This photo gives you an idea of the size difference of the original plate and the +3 extension. If you have a tactical muffintop, you will likely find the extension more comfortable in a magazine holder as it won't be treating you so much as the pilsbury doughboy.
No photos of the install, but it's again very simple. Hold the spring in place just to get things started, slided the extension on, flat side first, to line up the with flat back of the magazine, and slide on until the back edges are flush.
Now, before you through the set screw into place, confirm function. Load that magazine up with 17 rounds (if you have the SD40VE like me) and make sure they all go into place. Yes, those last three rounds will be TOUGH. Tougher than the last round was before (and I didn't think that was bad until this extention. If you can't get 17 rounds in, back it out to 14 rounds (or however many the 9mm takes) wiggle the extention forward and back a couple times and try again. In fact, I suggest doing this a few times even if you get the three additional in the first time to help make sure everything is centered.
Once that is all accomplished, apply a dab of blue locktite to the set screw and install just a lil past finger tight.
All in all, the only easier mod is a guide rod/spring change, but this one won't break your pistol.
There are Cons though:
-The sides of the extension are smooth aluminum. You'll probably want to add some grip tape.
-Magazine insertion on a closed chamber is TIGHT. That first round has a lot of pressure behind it, and could potentially cause a chambering issue, especially if you run a heavy recoil spring
-It's heavy. Ok, not the extension, it's light, but you may not realize how much weight three more .40S&W are until you load up and compare.
-It's longer. If you CC your SD, like I do, you won't likely want to run this as your primary magazine. The extra length with make the gun more difficult to conceal. I use it only on my backup magazine.
And other Pros:
-More Rounds! More is good, right up until it's too heavy. I also use my SD40VE for multigun competition, so if I can get by with fewer magazine changes, awesome
-No Rattle!!! The coolest unexpected change. We all know the magazine rattle when fully loaded. That goes away with the +3 Extension!