Tuning AirMaks Katran C Power Tuning, Initial Thoughts & Observations

[mtnGhost]

Spent the better part of my day on Saturday tearing down the .22cal FDE Katran C that I got from Michael. The appeal factor to me are the:

• Form-factor & compactness
• Light-weight
• Hammer system (should be very consistent)
• 300bar cylinder
• CZ barrel
• 16-shot mag (.22)

When I first looked at the rifle barrel, I took thr shroud off and flicked the barrel with my fingernail. Felt a nasty little vibration like a guitar string. Shot a few initial mags and wasn't too impressed with the groups at 50y (it definitely improved later on though!). So I tore it down and got a peek at the internals.

I like the overall simple, minimalistic design. The hammer concept is great and the valve looks pretty standard, but there are some things that I dislike:

1) Valve spring is incorrect for this system (neither strong enough nor is there enough compression!). I was just going to run a little PEEK valve seat to increase the compression, but the spring bottoms out. I wound up running a much stronger compression spring that doesn't interfere with valve pin / poppet travel.

2) Porting - small opening from the plenum > big low pressure area behind the poppet with a giant upper port > super tiny block transfer port (not enough "meat" left to expand it, and the only way to tweak it is through the top rail).

So I polished the valve body out with a special Dremel tool and accentuated the internal geometry for max airflow (like a Venturi)..



Somehow there was a missing o-ring in the barrel support (the threaded piece on the muzzle end of the block). Threw in a 14x2mm o-ring and that nasty vibration disappeared!


I cranked up the regulator, using AVS's tuning tips of ~5bwr per 1/8th of a full revolution (30-40bar per 1 rev). This unfortunately wasn't quite the case with this rifle, and I had some very weird valve lock and reg issues. I don't have an actual reg tester that fit this reg correctly, but I think 1/3 of a revolution adjusted it ~35bar. This plenum also has a big threaded brass piece that threads into the plenum and serves as an adjustable valve stop. I threaded it all the way in and it still interfered with the valve travel, so I just backed it out until the valve pin/spring could be fully compressed into the plenum.

CZ Barrel - I'm not a big fan of barrels held in by set screws (threads are king in my book). At least this one has a couple of flats though, so there isn't really any POI shift upon barrel reinstallation. You have to "feel" the screws bottom out flush before torquing them down (do not over torque to the point of bore distortion!).

I pushed a pellet through after cleaning out the bore and the chamber throat / lead in feels a little shallow. Might have to adjust this later. The choke measured out to roughly 3.5cm:

I got it buttoned up and shot another few mags. It was better, but there still appeared to be signs of harmonic issues affecting the accuracy. I don't like the barrel being detached from this shroud with the set screw / o-ring barrel support, so I decided to wrap the barrel tightly with a special dampening material which I then in turn wrapped with some special industrial fabric tape - adding just enough where the shroud is press fit tight over the materials:

The very last thing that I did was milled down the hammer. I noticed that there is a stop feature that prevents the hammer from making the valve travel ~2.4mm. I looked at the internals and found that I had lots of room to mill that down and I got about another 1.8mm of travel without concerns of the hammer striking the lever charging pin connector or other internals in the hammer's path. Here's the before photo of the hammer fully compressed against the valve (I'll upload the after later on)..

Results - I still have some fine tuning to do to see where the power maxes out, but this 280mm barrel is shooting 18.13gr JSB's at 926max thus far. I have it set to shoot them at 905 (3FPS ES). Need to refill my tank, but with a 198bar fill I can shoot 16 of the 18gr at 905 and my air cylinder gauge shows ~168-169bar remaining. The heavier spring definitely helps with this efficiency, but there's a lot to this overall tune.

It's grouping around 0.3" right now with my Nightforce NX8 1-8, so I would imagine it will be much better when I have time to test it with my 4-32.

 

[Bladebum]

I just picked up this exact gun and feel the mods you did are neccessary. It is wasting air imo and can be much more efficient, especially at the current fps (780 with 18gr) . What size spring did you put into the valve body? I considered just upping the reg pressure but my thought is the valve would still have too much dwell.

 

[Scott.t]

I just picked up this exact gun and feel the mods you did are neccessary. It is wasting air imo and can be much more efficient, especially at the current fps (780 with 18gr) . What size spring did you put into the valve body? I considered just upping the reg pressure but my thought is the valve would still have too much dwell.

Did you ever figure out the valve spring?

 

[mtnGhost]

I just picked up this exact gun and feel the mods you did are neccessary. It is wasting air imo and can be much more efficient, especially at the current fps (780 with 18gr) . What size spring did you put into the valve body? I considered just upping the reg pressure but my thought is the valve would still have too much dwell.

Did you ever figure out the valve spring?

I was running a heavier spring for different reasons, but the main efficiency issue with the compact (possibly others) is the regulator is configured to max out at ~107bar. To get the utmost efficiency with a short barrel the reg needs to be set around 130-140 (or higher for higher velocity).

A heavier spring is counter intuitive to the problem. It can cause the valve to dwell longer due to the resistance during the opening cycle (milli/nano seconds), whereas a lighter spring usually opens and closes quicker (higher pressure also speeds this up).

An aftermarket reg would be more efficient, but before I made a reg adapter for my reg tester I was inadvertently running my reg wide open to the cylinder pressure and manipulating the spring energy so that it self-regulated from ~200bar to 130bar (velocity started to deviate beyond 3FPS thereafter).

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Anyway, I would be extremely careful with tuning the Katrans that don't have a bleed valve. I had complete valve lock with mine last year and had to use a hammer and brass drift to depressurize it. The valve pin design is also not the greatest, the 2mm pin is like a needle that penetrates deep into the polymer poppet seal over time (my pin kept getting shorter and shorter until the valve lock incident where it was maybe 2mm away from poking all the way through the soft polymer seal).

 

[Scott.t]

I was running a heavier spring for different reasons, but the main efficiency issue with the compact (possibly others) is the regulator is configured to max out at ~107bar. To get the utmost efficiency with a short barrel the reg needs to be set around 130-140 (or higher for higher velocity).

A heavier spring is counter intuitive to the problem. It can cause the valve to dwell longer due to the resistance during the opening cycle (milli/nano seconds), whereas a lighter spring usually opens and closes quicker (higher pressure also speeds this up).

An aftermarket reg would be more efficient, but before I made a reg adapter for my reg tester I was inadvertently running my reg wide open to the cylinder pressure and manipulating the spring energy so that it self-regulated from ~200bar to 130bar (velocity started to deviate beyond 3FPS thereafter).

View this content on Instagram

View this content on Instagram

Anyway, I would be extremely careful with tuning the Katrans that don't have a bleed valve. I had complete valve lock with mine last year and had to use a hammer and brass drift to depressurize it. The valve pin design is also not the greatest, the 2mm pin is like a needle that penetrates deep into the polymer poppet seal over time (my pin kept getting shorter and shorter until the valve lock incident where it was maybe 2mm away from poking all the way through the soft polymer seal).

Interesting. I’ve been playing with the reg on mine, going higher and lower trying to tune my LB to shoot crosmans and/or 18.1 pellets in the 850-880 fps range. What I’m finding with mine is it does not like to be tuned low. Lower reg pressure with lower hammer spring tension results in wasting air which you can hear (burping sound). I can turn the reg up and get the speeds close to where I want but it’s very inconsistent. I have settled for shooting the 18.1 pellets at 930 fps, the setting is very consistent and efficient and still accurate at those speeds. It really likes the jsb monsters 25.4 pellets shooting them in the 930-940 fps range for 50+ fpe, the most accurate setup I have found for it, laser accuracy out to 100 yards

 

[mtnGhost]

Interesting. I’ve been playing with the reg on mine, going higher and lower trying to tune my LB to shoot crosmans and/or 18.1 pellets in the 850-880 fps range. What I’m finding with mine is it does not like to be tuned low. Lower reg pressure with lower hammer spring tension results in wasting air which you can hear (burping sound). I can turn the reg up and get the speeds close to where I want but it’s very inconsistent. I have settled for shooting the 18.1 pellets at 930 fps, the setting is very consistent and efficient and still accurate at those speeds. It really likes the jsb monsters 25.4 pellets shooting them in the 930-940 fps range for 50+ fpe, the most accurate setup I have found for it, laser accuracy out to 100 yards

Yeah that's a good speed for thr Monsters. Ever since I moved out into the country I've pretty much given up on the whole "efficiency" concept in terms of saving air, so all of my PCPs are power tuned.

My little Taipan Mutant has a 280mm CZ like my Katran C, but it's using an indestructible valve that I made for it a while back and I'm running a very heavy hammer configuration in it with a Huma reg maxed at 190 (250bar / 190cc cylinder). I had it setup to shoot 16gn JSBs around 930FPS, but I shot a squirrel with it in the shoulder and it just ran off .. so now it's setup to shoot the Monsters at 900FPS. It's getting substantially more shots than the factory Katran was when it was tuned for 780/16gn lol

I made a new valve for my Katran a few weeks ago, but it has a moderate leak and I just haven't had he time or motivation to finish it. The Katran form factor is a lot better thab the Mutant though .. much more ergonomic to shoot off hand than the Taipans.

 

[Bladebum]

Did you ever figure out the valve spring?

I never ended up messing with it, Instead I just upped the reg pressure to around 140-145 bar. This made the gun quieter with a nice snappy tone instead of a puff. Also, the power went up while shot count stayed the same. I get 3 mags with 18.13s going 815fps. I have no clue why airmaks would send these guns out with such low reg pressures. Mine is a newer version with the bleed off screw on the plenum and original reg pressure at 120 bar, it is very simple and fast to adjust the reg.