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Can you shimmy? (LPE Magazine shims)

If you've been in the Airsoft game for some time now, the chances are you've probably used an AEG before, and that probably means that you've heard all sorts of things regarding motor height, piston angle of engagement, and gear shimming, right? These are all things that when corrected result in a finely tuned gearbox and a usually well performing AEG, but if left unchecked could mean poor performance, usually without the person knowing that they are missing out on the potential gains all along until it's pointed out to them by an experienced user. So how does all this AEG talk translate over to our gearbox-less MK23? Well, you need to look at the Magazine and the Nozzle as your gearbox. Let's break it down: Your magazine consists of a gas reservoir (your battery and motor), feed lip rubber gasket /router (your gears) and directly connected to this is, your pistols nozzle (your piston). When these parts do not mesh properly, this is where you will start to see problems, with your FPS, nozzle return and trigger response, just like in an AEG. Correcting the tolerances of these parts so that they mesh correctly is key to having a stable FPS and removing performance/fps variances between multiple mags.


So how do we go about improving the function of our magazine and how it interacts with the nozzle? The answer is shimming. There are naturally several DIY ways to do this, using tin-foil or other materials found in your home, but by using them, you're potentially introducing user error and a point of failure. This is where the CNC shims from Lees Precision Engineering could be what you need. LPE produce high quality upgrade and replacement parts primarily for pistols, and in this case, they offer magazine shims specifically for Tokyo Marui magazines, which we have found to be compatible with the MK23.

These shims are ideal at fine tuning and altering the magazine router to gas nozzle seal by raising the router height within the magazine housing; with better seal equating to better performance and consistency. Multiple shims can be installed and there are 10 shims provided per pack. We purchased a pack and set about investigating their effectiveness!


Below, I'm going to go through the procedure on how I fitted and tested these shims, my ideal aim here was to stabilize the fps across several of my magazines to try and match the fps variances between them to achieve a very consistent setup.

Shake those hips! (We'll assume you know how to remove the feed lips for the purpose of this article, if you don't know, check out this link)

1. First, I made sure that each router was well seated by pressing each one down firmly, I then fully charged my mags with gas (propane), labelled my mags 1 - 5 and loaded my go to weight of BB's (a few 0.48's in each) and took a chronograph reading.

2. I noted down the average chronograph figure down for each mag and took the highest performing magazine as my benchmark.

3. I added one shim under the router and tested a few dry fire shots to see if the added height introduced the sticky trigger issue - it did not so I added an additional shim and this did introduce sticky trigger so I reverted back to the one shim.

4. I took a new fresh chrono reading on the altered mag and noted this as the figure I was aiming for with the other mags. I repeated the shimming process, adjusting the amount as needed as some mags needed two shims whilst others needed just the one shim added.


The procedure of fitting the shim is really simple and it's not necessary to discharge the gas from the magazine to do so as the valve below the shim will hold the gas pressure and prevent release.

The steps of fitting are as follows:

With the feed lip and gasket removed, install the shim in the magazine router housing and re-fit the BB follower with spring, then the mag router and feed lips, then press in the pin from right to left making sure that the knurled end is going through last.

Here you can see one shim fitted and a nice shot of the valve (in the correct flow position) with a window facing upwards.

Note: from here you can also check your valve outlet alignment. Best perfromance by having a window facing upwards by tightening (best) or loosening (2nd option)

Shimmy shimmy ya!

So with all 5 test magazines altered with correct shimming height, let's summarize my results below.

- I left my best performing mag without a shim as it actually pushed it over the joule limit and was averaging 216fps (on 0.48g) without a shim fitted.

- One magazine needed only one shim to bring it from 206 up to 215fps and two other mags needed two shims to raise them from 196fps up to 216fps and 219fps.

- The fifth mag was also left as it was at 215fps.

For science, I added three shims to one of my mags and it raised the fps from 196fps up to 240fps but produced sticky trigger issues with the trigger not returning to rest position without manual reset. Also way over the 222fps limit on 0.48g BB's (if you're wondering how to get to 500 FPS on a 0.20g BB, this is pretty much how you'll want to do it). The net result is clear to see, the FPS figures from all 5 magazines have been brought in line to a more consistent level, with each mag being within a 4 FPS variance. At less than £1 per shim, this is a very cheap an effective way to achieve a more consistent setup across all of your magazines, and a sure way to improve your shot to shot accuracy.

At the time of review, I did not have an ASG / Clone magazine available to test the fitment of the LPE shims, however, knowing that the tolerances of the gas router housing and the router itself are different on the ASG when compared to the TM, it's possible that these shim's may be incompatible with the clone magazines. We're waiting on obtaining a Clone magazine to verify our results further, but until now, be aware of this if you're considering a purchase. Besides the potential incompatibility with the clone magazines, I'd struggle to find a negative argument against using these shims with my MK23, there is only a gain to be had for a very minimal amount of installation effort. As a final word, please take in to account that each MK23 and magazine will differ from user, due to setup and factory tolerances. Your results may vary and differ to my findings!

If you're interested in trying the LPE shims out yourself, you can find the product information available here. Or head over to their main website link:

Our community currently has a discount code which you can apply at checkout, just use "MK23Owners" for 10% off.

Happy shimming!