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Install and initial review: KNS AK-Pattern Adjustable Gas Piston - Galil ACE .308


Geth Prime
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Picked up a used but excellent condition Galil ACE .308 last year (thanks, Marv). Runs excellent on everything I've fed it, but, like many AK-pattern rifles, it is very enthusiastic about brass ejection. I've been wanting to add a can (suppressor) but adding a gun muffler to an already fairly over-gassed baseline...well, lets just say it 'enhances' both things :)

To give you an idea of how far the Galil ACE likes to throw my brass, here is a pic:

Brass Pattern.jpg

The Dam police are getting a bit irritated at the random rainstorms of 308 brass and are getting less cooperative about my brass hounding on their turf (its my brass and I want it back :) ).

Since I absolutely must be able to run a can, and do not want to deal with the terror tales (see I used an alternate phrase there) from other Galil ACE .308 owners, I decided to see if KNS had one of their adjustable gas pistons for this rifle. Lo and behold they do: part number AGP-A-8.

The part arrived a few weeks back and instructions in hand (okay, 'in iPad') I went about installing the new piston. In case anyone else with an ACE decided to go about doing this, I took some pics of the process (it is the same for all models except the 'pistol') as I went along.

The Retail Package:

The package includes the assembled piston, piston head, and 'star guide', as well as a second star guide (more on that later), a couple of roll pins, a sticker, some basic instructions, and staples that will bite you. Damn things.

Sorry about the glare, a photographer I am not.
KNS Piston 1.jpg

The Piston:

The piston assembly is composed of three parts: The piston rod, the piston head, and the star guide. The piston head includes the ratcheting gas adjustment collar, which (at least on this model) has 60 very-positive 'clicks' of adjustment from fully closed, to fully open.
Stock piston and KNS piston for comparison
KNS Piston 2.jpg

The 'star guide' is not included on all models of the adjustable piston, however, since the Galil has one in it stock form, KNS include it in the design for their piston. What is it? I'll quote the KNS manual:

KNS install manual said:
A few AK models feature a 'star guide' on the piston to align the piston head inside the gas tube. If your gun has this feature, be sure to request it when selecting & ordering your piston, as some models were made both with and without this part, and it may not be included by KNS by default. When it is not provided, a small blanking 'dummy collar' will be installed on the op-rod where it attaches to the piston head.
KNS includes two different sizes of star guide in the package. The 'large' one is installed from the factory, with a slightly smaller unit also included. They recommend using the large diameter one unless it has clearance issues when installing it back in the rifle. The smaller of the two can be swapped in by driving out the roll pin (see above pic), unscrewing the piston head, pulling off the large star guide, replacing it with the smaller version, screwing the piston back in, then driving the roll pin back in place.

I measured mine to see what the differences were:
KNS Piston 5.jpg

As you can see, the version that comes installed on the KNS piston is a bit larger, however, I did check fitment on the piston head in the gas tube and gas block and it seemed to have good clearance (more on that later). Note: I did measure the small star guide and it was slightly undersized from the stock version (and the pic turned out very blurry, sorry).


Now it was time to remove the stock piston from the Galil.

The first thing to do is get a measurement from the end of the piston head to the end of the piston carrier 'nose'. You'll need that for a reference when installing the new piston. Once you have the measurement the stock piston can be removed.

Here it is actually easier on the Galil than on some other AK-pattern rifles as the Galil uses a spring pin to secure the piston rod in place, rather than a rivet like some others. That makes removing the stock piston as simple as tapping out the stock spring pin with a roll pin punch and unscrewing the piston.

KNS Piston 3.jpg

Items of note:

Once you have the stock piston removed, you'll notice two differences on the threaded portion between the stock and KNS pistons:
  1. The hole on the stock piston is quite a bit larger in diameter than the hole on the KNS piston. Per the instructions KNS did this intentionally and notes that "the threads carry all the piston force, and the pin simply prevents the piston from turning".
  2. The stock piston has a single hole through the threaded portion of the piston, while the KNS has two holes perpendicular to each other. This was done to allow for a more 'precise' adjustment for the overall length, with a hole every 90degrees.
KNS Piston 4.jpg
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Geth Prime
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Installing the KNS Adjustable Piston:

To install the KNS piston, screw the piston into the carrier all the way, then back it out until one of the pin holes align with the opening in the carrier. Take a measurement from the piston head to the nose of the carrier. According the instructions you want to be within 0.03" of the stock measurement you took earlier. If it is too short, spin it out the next hole, measure again. Once you find a close match, you may then tap in the supplied roll pin using a roll pin punch. Properly installed, the roll pin should stick out of the threaded portion on both sides, but not extending outside of the carrier hole. Just like the stock piston, it will wobble a bit. It is supposed to.

KNS Piston 6.jpg

Adjusting the Piston:

KNS has a wordy section on adjustments, but I'll give a more condensed version:

  1. Start with the piston in the fully closed position. This is to test as close to 'stock' or original condition as possible.
  2. With a full magazine of known 'good' ammo, test for proper cycling (load, fire, extract, eject, load, repeat).
  3. Assuming that works correctly, KNS suggests making small adjustment in 5 click increments and continue until the firearm fails to cycle properly (note: each adjustment requires you to remove the carrier assembly, make the adjustment, then reassemble your rifle).
  4. Next they suggest increasing the gas by 2 clicks until the rifle cycles properly. This is considered (in their words) the 'borderline' operational point. You can now make adjustments for various ammo loads (powder and bullet weights), increase gas a few clicks for reliability, reduce gas for suppressed use, etc.
KNS Piston 7.jpg


Now, on with the review...

KNS Adjustable Piston - At the Range:

On my first outing to the range I brought along a few different types of ammo to try (and tools, because you will always need a tool when you've left it at home, believe me):
- Magtech 147gr M80 ball (7.62x51)
- MFS 145gr FMJ .308 (Zinc Plated Steel Cased)
- Federal Power-Shok 150gr SP .308
- Federal Power-Shok 180gr SP .308
- Australian Outback 168gr SMK .308

Starting with the piston in the fully-closed position I loaded up 3 rounds of Magtech and proceeded to slowly fire, noting the brass ejection pattern and distance. The Galil functioned without issue, however, my brass was now falling about 2-12' away, rather than somewhere in Arizona. I was both pleased and puzzled. Fully closed was a huge difference vs stock. Just to be sure, I loaded up two more 3rd mags, and then 2 fully loaded 10rd mags, all with the same results. Brass falling between 2 and 12' away, give or take. Loaded up some of the MFS .308 with basically the same results.

Now, to be clear, there is one small line buried in the FAQ on the two different bore sizes:

KNS FAQ said:
"Since the large bore piston has greater internal volume at rest, it is common for the firearm to “short stroke” {referring specifically to an Arsenal 107CR} even with the piston fully closed and muzzle booster installed. To counter this - a small bore head is used to push the adjustment band into a usable range - despite the natural tendency to use a large bore piston with a .30 bore."
KNS Piston 9a.jpg

With the fully-closed position 'basic' function tested (as odd as it turned out), I proceeded to remove the carrier assembly, make a 5 click adjustment, reassemble, and loaded up a few more 3rd mags.

The results were...curious. Now my brass was pegging my support hand (ouch!) and dropping about a foot in front of me, or just kinda' rolling over and playing dead right next to me. Okay, that is as odd and concerning as it sounds at a mere 5 out of 60 clicks.

Opened it up to 10 clicks and got a stovepipe on the first round. Hand cycled, fired another, it picked up the next round then stovepiped the next. Tried it with some MFS, and some Federal 150s, with similar results. I then proceed to try 9 clicks, 8 clicks, 7, 6...with the following results:

KNS Piston 8sml Notes.jpg

Back at 5 open clicks I decide to run a few full mags through in quick double taps. Couldn't get through a full mag without a stovepipe. Sigh.

Click down to 4 open clicks with similar results (can mostly get through a mag, but still gets a stovepipe). At 3 open clicks I was able to get through 4 full mags without issue, brass (or zinc plated steel) cases landing 2-6 or so feet away.

Since I had my tools with me I decided to swap the stock piston back in to see if something else might have gone wonky whilst I was fiddling about. BANG, launches brass out of the bay (and likely out of the state given the velocity at which it attempted to attain low earth orbit).

Swap the KNS back in (it really is a pretty quick process), close down the piston, bloop, brass dropping just like before 2-12ish feet away (most at 1 o'clock, with usually one case per mag that has to be the iconoclast, flying back towards 5 o'clock, 10-12' away.

I fiddle about a bit more then decide to go home. The sun hates me and I reciprocate the feeling during summer.

Contacting Support:

Once home I draft an email to KNS support inquiring about the very, very large difference in brass ejection, and the utter inability to vent the gas more than 3 clicks (nicely worded and detailed; I'm not an animal, no matter what my wife says).

KNS gets back to me early on the next Monday with a short clarification: "...the addition of the piston system will cause a slight reduction in gas pressures even when completely closed. The factory piston has a solid face were ours has a hole in the end. "

Yes, thank you. I read that (and I did thank them for getting back to me so soon. Stellar.), however, my experience is not 'slight' by any means, but rather drastic in the change. YUGE as a certain person might say :)

The next reply (also blazing fast, thank you KNS) was a bit more detailed: "The AK Platform is not an exact science. For this reason, we have done as much research as possible to find what piston and op-rod works with what rifle/pistol. For an over-gassed 7.62x39 rifle all testing has pointed to using a large bore piston, but sometimes there are those AKs that are not near as over-gassed as the others. Those may actually need a small bore piston to function correctly. It sounds like your rifle falls into that category."

The support person then offered to swap my piston head for a small bore head. Which was a great potential resolution. I pondered that a bit; however, before accepting the offer I wanted to do more testing to see how reliable the 'fully closed' position was with several 10 and 20 round mags fired at a rapid pace. That left me at this morning.

Out before the sun as is my usual want, I setup to test (and waited a bit since there were Eldorado Championship RVs and trailers camping out in the public area open field), then proceeded to fire off rapid double taps through several 10 and 20rd mags of various ammo (and I really do not want to think how much in covid ammo inflation dollars I burned through for that...heh). Ate everything, cycled, no issues, had a nice line of brass starting about 2' away and running through about 15' away (at 1 o'clock, except for those few hippie rounds that just had to be different and fly back at 5 o'clock...).

So, where are we?

Given my testing this morning, I 'think' I have a potential baseline with the piston fully closed (pre-can) that seems to function without issue. I am a bit wary of how it will perform when really dirty, but that's about 450 rounds without cleaning so far, so something to watch. My next steps will be to mount a SilencerCo muzzle adapter so I can throw on a can. If, as I suspect, I'll be able to actually dial gas down then this exercise will not have been for naught. Otherwise, I'll likely take up KNS on their offer to swap piston heads so I have a more 'energetic' baseline to start with. That is my primary concern at this point.

Once I get the muzzle adapter mounted and back out to the range with a can I will update this thread.
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Geth Prime
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Thanks, Joe (y)

I figured somebody out there might have an AK-pattern rifle they are considering using a can with, or are already using a can and are experiencing massive over-gassing. My Arsenal SGL21-61 is a very good candidate for this. I may pick one up for that rifle if all goes well with the rest of this experience.

KNS has something like 15 different versions of this for various AK-pattern rifles.
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Staff member
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Very well done sir. You might consider a second career as a gun writer, a good many articles in Guns and Ammo, Guns, Shooting Times, etc. are not nearly so well done as your post above.


Geth Prime
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Thanks, guys. I am glad it is being positively received and hope it helps someone else in both their buying decision and install process.


Grumpiness incarnate!
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Good review! I've read professional reviews that weren't as informative or fluently written. I have a 5.56 Ace and have been thinking about buying this setup, so will be looking forward to updates.