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All Springer/NP/PCP Air Gun Discussion General => PCP/CO2/HPA Air Gun Gates "The Darkside" => Korean Gate => Topic started by: oldpro on March 12, 2021, 04:23:11 PM
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I picked up a Evanix AR6K from Airgun Pro shop and love the little light weight mechanical Semi Auto action!!! Such a simple design and really accurate too! You know me though nothing stays stock for long. The gun works in single and double action so drawing back the hammer by hand nets me 770 fps with JSB hades 22 ammo 15.89 grain and in double action in the mid 550s. My goal is to try and get the double action up to or near the single action speed so heres what Im doing. Porting the valve and action to .165 currently at .130 next open up the valve end and throat then build a custom one off balanced valve!
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Here's some pics of my progress. I'm about done and getting ready to run it over the chronograph. These are stock pics of valve and breech and link to youtube video of gun in question. https://youtu.be/_TNJmsIlfQ0
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Here is the balanced valve mods and hammer mods with power adjuster.
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Ok first test over chronograph done averaged 925 in single action and 800 in double action. The spread not so good so I'm going to now make a valve limiter built next to the velocity adjuster on the hammer so I can adjust valve travel. More to come. getting there. Also I think I can make a lighter hammer spring and reduce pull weight in double action.
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Saw a YouTube video by Expert Airgun Reviews. Seems really good. Looking forward to seeing what you do with it.
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Travis is the slotted brass screw on the rear of the receiver a hammer spring tension adjuster?
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Travis is the slotted brass screw on the rear of the receiver a hammer spring tension adjuster?
There are No factory adjustments of any kind on this gun. So no sir.
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Thanks, and too bad! This is one of the straight-barreled guns (no shroud or baffles) used during LDC testing and it would have been good to easily adjust it up over 30fpe, but apparently you are solving this as we speak... 8)
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Thanks, and too bad! This is one of the straight-barreled guns (no shroud or baffles) used during LDC testing and it would have been good to easily adjust it up over 30fpe, but apparently you are solving this as we speak... 8)
How did you like it?
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Finally someone doing mods on the same rifle I have. Following along even though I have no way of doing this myself on my own gun.
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Watching; I like the swinging hammer concept of this design; interested to see what you can do with it. :D
Jesse
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hit 985 fps on high setting with .22 JSB Hades! averaged over 950 fps for two mags on high! WOOF!!! This is going better than expected.
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I love the AR6 platform, wish Evanix kept the all line, up to .30 in production.
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I love the AR6 platform, wish Evanix kept the all line, up to .30 in production.
I do have a .25 barrel and mag with this gun. Ill swap them and see what she will do. No way in stock form it could push a .30 and even a .25 is way under powered in stock form. The platform is awesome I really like it a lot!!!
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I thought stock form was a little snappier than what your results were...
https://youtu.be/CkxMT2rTonw
Cool gun on my must have list for sure !
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Uncle Mike's 24580 Hammer Extension for Marlin Lever Actions (1983 and Later), Black https://www.amazon.com/dp/B001MQM6WQ/ref=cm_sw_r_cp_apa_i_F6709GCW7RBQMMPH32C5 (https://www.amazon.com/dp/B001MQM6WQ/ref=cm_sw_r_cp_apa_i_F6709GCW7RBQMMPH32C5)
If you need to add a little extra mass to the hammer. These are cheap- only ads a little weight and makes it easier to cock with a scope.
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I love the AR6 platform, wish Evanix kept the all line, up to .30 in production.
I do have a .25 barrel and mag with this gun. Ill swap them and see what she will do. No way in stock form it could push a .30 and even a .25 is way under powered in stock form. The platform is awesome I really like it a lot!!!
Yeah not the K model,
But the full sized models had some serious power, Rich has a .30 Bottle version that I really wanted to buy but he just asks to much money unfortunately
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Thanks, and too bad! This is one of the straight-barreled guns (no shroud or baffles) used during LDC testing and it would have been good to easily adjust it up over 30fpe, but apparently you are solving this as we speak... 8)
How did you like it?
I liked it, and Still like it! (so I still have it) :D
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Thanks, and too bad! This is one of the straight-barreled guns (no shroud or baffles) used during LDC testing and it would have been good to easily adjust it up over 30fpe, but apparently you are solving this as we speak... 8)
How did you like it?
I liked it, and Still like it! (so I still have it) :D
Oh, cool! Seemed pretty awesome from the videos. I was tempted, but couldn't justify getting one. Good deal on them, too.
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Found a bit more in it. 1000 FPS with Hades .22 out of this little thing. Accurate as heck too!
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Found a bit more in it. 1000 FPS with Hades .22 out of this little thing. Accurate as heck too!
Whoa, that's crazy! 8)
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Travis ,
You really need to try some .221 “ bullets in it. The RWS bullets that come from Krale shoot excellent in those barrels even as low as 500 FPS. They also really like the JSB monsters. I like the balanced valve you did on that one. I went with a cobra set up and a heavier hammer to make up the loss. One of those guns I’ll never get rid of. It’s been opened up probably close to 30 times and I still haven’t ever had a problem with it.
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I started to say that Hobbyman2007 has quite a bit of experience with modding this platform now. ;)
Mike/Knife
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Travis ,
You really need to try some .221 “ bullets in it. The RWS bullets that come from Krale shoot excellent in those barrels even as low as 500 FPS. They also really like the JSB monsters. I like the balanced valve you did on that one. I went with a cobra set up and a heavier hammer to make up the loss. One of those guns I’ll never get rid of. It’s been opened up probably close to 30 times and I still haven’t ever had a problem with it.
My sentiments exactly Denis, there is really nothing else like the AR6K available now and it is solidly precision machined to reliably last a long time.
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Amazing to see what you’ve done with this platform. I especially like that you added a balanced valve to allow for lighter HST.
What’s the shot count like with all the mods?
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So Travis, what is the final outcome of your mods to this AR6K ? Is it still operating. I find your work with this rifle very interesting and would like to know more about your results long term.
Regards,
Dan
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I love the AR6 platform, wish Evanix kept the all line, up to .30 in production.
I do have a .25 barrel and mag with this gun. Ill swap them and see what she will do. No way in stock form it could push a .30 and even a .25 is way under powered in stock form. The platform is awesome I really like it a lot!!!
Yeah not the K model,
But the full sized models had some serious power, Rich has a .30 Bottle version that I really wanted to buy but he just asks to much money unfortunately
i thought you dont like bottle guns? bottle ar6 are not common, maybe his price reflects that
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I love the AR6 platform, wish Evanix kept the all line, up to .30 in production.
I do have a .25 barrel and mag with this gun. Ill swap them and see what she will do. No way in stock form it could push a .30 and even a .25 is way under powered in stock form. The platform is awesome I really like it a lot!!!
Yeah not the K model,
But the full sized models had some serious power, Rich has a .30 Bottle version that I really wanted to buy but he just asks to much money unfortunately
i thought you dont like bottle guns? bottle ar6 are not common, maybe his price reflects that
Correct I don't, but was going to overlook it just to get an AR6 .30,.....also I would have cut barrel and shroud to make it level with the bottle if I did ever get it, and that to me makes it appealing to the eye.
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From another forum , I was able to find a 700cc bottle that I’ll be adding to my K550 ( same action ) this should bring the bottle to just behind the barrel end. The other thing is I’m going to tune this to have as much power as possible. I’m shooting for as close to 80 FPE or over . I cast some NOE bullets from the 225-39 mold that are extremely accurate from this gun out waaayyyyyy past the 100 yard mark. One of my favorites for sure , never had a leak , barrel is threaded into the action , very quick follow up shots if needed. If I could get my hands on the K version in .25 I’d pick it up in a hurry. Better yet , if Rick would ship that .30 to Canada it would have already been sold.
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Biggest mistake EVANIX made was to drop the AR6/K550 lines,....such a cool, unique gun
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Tore into the rifle last night to prep for the 700cc bottle. Opened up the transfer port and barrel port to .210” . Then replaced the jet size on the cobra valve to one that was slightly bigger for faster opening. Put it back together and hit 83 fpe shooting the 41 grain RWS bullets and 86 fpe with the 39 grain cast. 955 and 990 FPS . This was peak velocity at 185 bar. This should give a real good shot string from 200 bar to 150 bar.
I’m absolutely certain the AR6 k is capable of the same power. With a few mods .
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Denis, what kind of accuracy/groups are you getting with those bullets? I have a bottle gun conversion of AR6 pistol ongoing and I haven't quite decided on the barrel configuration - I have 10" original barrel, 14" Crosman barrel and 27" FX liner that I could use.
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Denis, what kind of accuracy/groups are you getting with those bullets? I have a bottle gun conversion of AR6 pistol ongoing and I haven't quite decided on the barrel configuration - I have 10" original barrel, 14" Crosman barrel and 27" FX liner that I could use.
I believe he said it shoots “extremely accurate” wayyyyy past 100 yards. i would expect 1” at 100 yards to be “extremely accurate.”
I also have a rifle version i wanna convert into regulated bottle fed.
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Tore into the rifle last night to prep for the 700cc bottle. Opened up the transfer port and barrel port to .210” . Then replaced the jet size on the cobra valve to one that was slightly bigger for faster opening. Put it back together and hit 83 fpe shooting the 41 grain RWS bullets and 86 fpe with the 39 grain cast. 955 and 990 FPS . This was peak velocity at 185 bar. This should give a real good shot string from 200 bar to 150 bar.
I’m absolutely certain the AR6 k is capable of the same power. With a few mods .
Pictures ? pictures ? pictures ? :) :)
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Denis, what kind of accuracy/groups are you getting with those bullets? I have a bottle gun conversion of AR6 pistol ongoing and I haven't quite decided on the barrel configuration - I have 10" original barrel, 14" Crosman barrel and 27" FX liner that I could use.
I believe he said it shoots “extremely accurate” wayyyyy past 100 yards. i would expect 1” at 100 yards to be “extremely accurate.”
I also have a rifle version i wanna convert into regulated bottle fed.
Pistol version is actually a good candidate for bottle conversion as the original airtube will then become a large plenum and you can use regulated paint bottle in front of that. 1" groups at 100M would work for me.
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Here’s my old Avatar pic . This was shot at about 100 yards with this rifle with cast 225-39 bullets sized to .2216”. I’m hoping to get out and see if they shoot as well with the added velocity. I also have the same mold decked down to cast them at 31 grain . This was done by one of our own Knifemaker. The RWS bullets shoot also very well as they ship and I’ve never sized them. As for pics of the rifle , it haven’t changed since I added the hammer sights to it. It can be seen in my review with mods.
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I recently acquired Travis's ( Oldpro ) Evanix AR6 with the balanced valve. I am to sell it on his behalf being he is currently doing a permanent move to Texas.
If interested ... PM me privately. Eventually I'll get it up on the F/S area otherwise.
Scott S
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Here’s my old Avatar pic . This was shot at about 100 yards with this rifle with cast 225-39 bullets sized to .2216”. I’m hoping to get out and see if they shoot as well with the added velocity. I also have the same mold decked down to cast them at 31 grain . This was done by one of our own Knifemaker. The RWS bullets shoot also very well as they ship and I’ve never sized them. As for pics of the rifle , it haven’t changed since I added the hammer sights to it. It can be seen in my review with mods.
That's accurate enough for me. I was mainly worried about the accuracy effects of bullet hitting the rifling at speed when it exits the magazine. I guess I'll be using the FX liner then, it did 80+ fpe with H&N slugs in my BSA so it should give good power in AR6 as well.
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RKR,
If using an FX liner , you will need to have a longer taper on the leade into the rifling. I’m not sure what angle but I can tell you that it’s quite a bit different than the usual abrupt taper on rifles with a probe. Same can be said for the Leshiy 2 that shoots from the magazine.
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Would love to find a Windy City .357 and put one of those 700cc bottles on :)
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Would love to find a Windy City .357 and put one of those 700cc bottles on :)
I am working on a Windy city in .257. its an air hog. 1.1L bottle sounds better lol
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Would love to find a Windy City .357 and put one of those 700cc bottles on :)
I am working on a Windy city in .257. its an air hog. 1.1L bottle sounds better lol
I like air hogs :) I never go for shot count the 700cc bottle would be more for looks ;)
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I did have a Windy City MK1 with the steel magazine. Nice gun for sure but the transfer between the bottle and valve was pretty restricted and I couldn’t make the power I wanted. I even tried to convert it to .172 cal . I eventually sold it. I’m sure Evanix did something to the drop block for the bigger calibers. That would be a good set up and lots of air to boot.
Wonder how much Travis wants for the AR6K ? That’s a nice little rifle , perfect for day trips .
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The 257 windy city max out at 140 foot pounds but I ended up tuning it down for 124-125 foot pounds for five shots. would be ideal .172 but probably air hog too.
there is a restriction point. i milled it out.
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I did have a Windy City MK1 with the steel magazine. Nice gun for sure but the transfer between the bottle and valve was pretty restricted and I couldn’t make the power I wanted. I even tried to convert it to .172 cal . I eventually sold it. I’m sure Evanix did something to the drop block for the bigger calibers. That would be a good set up and lots of air to boot.
Wonder how much Travis wants for the AR6K ? That’s a nice little rifle , perfect for day trips .
Bigbores have restricted air feed as well, that bottle connector has 6 tiny holes around it in the middle for the air to flow through to the valve. There's no way to make big power with that setup so mods are needed. .257 AR6 would actually be quite interesting, 257420 bullets should fit in a modded magazine so if it could make 150 fpe ...
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I have the ar6 standard version that now has a leak from the check valve when filling so I need some orings. Rather than just replace the orings I’m thinking about having it tuned. I love shooting the gun but also wish I had an extra mag or two. Any suggestions on who to send it to.
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Will Piatt @ Saddlemountain gunsmith
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Will Piatt @ Saddlemountain gunsmith
Yup .. Will is the guy for that !!
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A question, what's the easiest way to take the valve block apart in these guns? I guess the front part screws off or pulls of and the transfer port is locking it in place. Can you pull off the brass transfer port or does it have to be drilled out?
Edit, TP pulls out and the front part is threaded on - it's just bloody tight. I had to put in a 3 jaw chuck to get it unscrewed. 3.5mm valve stem in a 5mm valve tunnel which equals 3.5mm ports the gun has, no wonder the power is restricted!
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Did you guys drill out the rear holes on the magazine/drum when tuning and if so how far did you go? They are 4.7mm in my magazines and it looks like about 5.0mm is the max we can go if we want the spring loaded ball bearing to align the magazine correctly. That would then mean that there's no point in going larger than 5.0mm ports, unless of course we use bigger ball bearing ...
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Riku , I went with 5mm like you’re seeing. My transfer port is at 5.1 between the valve and the action and the other port is at 5.1 just the same. Here’s the K550 with the 700cc bottle installed . Was tough getting the old 480cc off but I managed. 220cc of extra air is awesome. Especially on an unregulated rifle where the real sweet spot is between 180 and 150 bar. The rifle is quite long but I think it looks a lot better too .
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wwhere do i get my hands on one of those 700cc bottles
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Love the looks of that gun !!!!
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The bottles can be bought from http://alsafecylinders.com/ (http://alsafecylinders.com/)
You need to use their contact form then send money though alibaba pay. It took less than two weeks to get to me in Canada.
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Thanks Denis. I checked the ball bearing with washers and a washer with 5.5mm hole no longer pushes the ball in, so 5.2mm ports should still work. Unfortunately long 5.2mm drill bits needed to drill the port to the magazine are nowhere available locally so I'll go for 5.0mm ports for now. Larger ball can not be used, this one is as big as can be fitted in the breech. What did you do for the actual transfer port pieces? They have 6.0mm brass inserts and plastic sleeve around it. I'm not sure if they'll hold when bored out so I was thinking of making them out of PTFE.
The 2.0mm stem valve is done, valve closing force is reduced by about 70% and with PET-P poppet and reduced seat diameter the force keeping it shut is also reduced. I need to go shopping for a long 5.0mm drill bit next:
(https://live.staticflickr.com/65535/51671569751_dc32752c0d_k.jpg) (https://flic.kr/p/2mJ3itD)20211111_105806 (https://flic.kr/p/2mJ3itD) by abbababbaccc (https://www.flickr.com/photos/11843711@N08/), on Flickr
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PFTE should work , I used some 5/16 synflex air line tube that I had from my days as a Highway tractor mechanic. I did have to turn it down on the lathe using coarse sand paper. Good work on the valve . Because I’m using a cobra set up I’m fairly certain I could get more power out of it . But I’m very happy with where I ended up.
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Starting the AR6 pistol bottle conversion. I have a 380cc PB tank with 2200 PSI regulator, that's 150 bar. I started by testing the power with the pistol's backyard tune that gets two magazines worth of shots. 30 grain H&N slugs fly at 560 fps using 14" barrel in the gun, that's 21 fpe so a good starting point. 10" barrel should do about 520 fps and 18 fpe which is still rather useful power for backyard pistol. This tune has restrcited transfer port and very light hammer spring so it's time to see if I can up the power a bit.
As noted above I took the gun apart and found out that TP is 3.5mm in size.
Fast forward a bit, 2mm stem valve was made and ports were opened up to 5mm. At 150 bar and with 30 grain slugs 14" barrel and still that loose hammer spring in there the gun is now shooting at 780 fps which makes 41 fpe. That's not bad for a little air pistol and twice the power I started with :D I think it's time to see what happens with heavy hammer spring. There's also the 26" barrel that I can use for some 120 fps extra speed. Lots of tweaking to do ...
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Hammer springs:
(https://live.staticflickr.com/65535/51676266998_f0173523e3_k.jpg) (https://flic.kr/p/2mJsnNE)2021-11-13_11-57-44 (https://flic.kr/p/2mJsnNE) by abbababbaccc (https://www.flickr.com/photos/11843711@N08/), on Flickr
I took out the short spring and installed the original long spring, they are real PITA to change. There was no increase in speed which indicates that I'm blowing air out of the barrel long after bullet is gone. It also points to the barrel length and port size being the limits here. I'm considering 5.3mm ports, I found a source for long drill bits and if I replace the ball bearing that orients the magazine with a shaped cylinder it should work with magazines drilled to 5.3mm opening at the rear. As it is now the magazine is stiffer to rotate as the ball bearing goes in further and the angle of the bearing vs. magazine is steeper. That 5.3mm porting would give me 12% more flow area so possibly 46 fpe with 14" barrel and theoretical 60 fpe with 26" barrel.
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RKR,
If using an FX liner , you will need to have a longer taper on the leade into the rifling. I’m not sure what angle but I can tell you that it’s quite a bit different than the usual abrupt taper on rifles with a probe. Same can be said for the Leshiy 2 that shoots from the magazine.
I took a plasticice "cast" of that "chamber" and there's 9mm long straight part at groove size and then leade in to rifling. I think I'll copy that and use the FX liner and see how it works.
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The Bighorn might fill the gap in the unavailability of a bottle Evanix. Allows for a slightly heavier hammer and a lot more spring due the lengthened action. I'm running Benjamin Bulldog springs in mine with a 60gr hammer. 20 shots at 85fpe in 25 with a 22.8" barrel. Perfect candidate for a 28" TJ 257 barrel swap. Is already full bore ported in 25. Comes with a 500cc 250bar bottle.
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The Bighorn might fill the gap in the unavailability of a bottle Evanix. Allows for a slightly heavier hammer and a lot more spring due the lengthened action. I'm running Benjamin Bulldog springs in mine with a 60gr hammer. 20 shots at 85fpe in 25 with a 22.8" barrel. Perfect candidate for a 28" TJ 257 barrel swap. Is already full bore ported in 25. Comes with a 500cc 250bar bottle.
It's the unique exposed hammer action and the long rotary magazine that make the AR6/K550 rifles so appealing,
Otherwise yes you right, those Krals definitely can replace the Rainstorm/Windy City lines once you ( Troy Hammer ) work on them and with them aftermarket magazines made in the UK
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The Bighorn might fill the gap in the unavailability of a bottle Evanix. Allows for a slightly heavier hammer and a lot more spring due the lengthened action. I'm running Benjamin Bulldog springs in mine with a 60gr hammer. 20 shots at 85fpe in 25 with a 22.8" barrel. Perfect candidate for a 28" TJ 257 barrel swap. Is already full bore ported in 25. Comes with a 500cc 250bar bottle.
85 fpe is nowhere near enough for .257. You want at least 140 fpe or preferably 160 fpe to get those 257420 & 257388 bullets up to speed. That's doable with Evanix Blizzard and Sniper X2 bottle gun but if Kral only makes 85 fpe in .25 it seems rather low for that. Then there's the issue of barrel support, Bighorn has rather short length of breech that supports the barrel and the rest is free floating. When you attach a 28" or rather a 30+" .257 barrel to that it will whip and quite a lot. In my Blizzard which seems to have longer breech support for the barrel than Bighorn, I had to install a barrel band with no o-rings to get good accuracy. And as for good accuracy I'm referring to sub 1" groups at 100 yards. If I was to build a .257 from scratch I'd seriously consider Kral as a starting point but it really needs to make more than 85 fpe in .25 to make it feasible. If there's not enough hammer strike you can always use a 2mm valve stem to make more valve dwell.
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The Bighorn might fill the gap in the unavailability of a bottle Evanix. Allows for a slightly heavier hammer and a lot more spring due the lengthened action. I'm running Benjamin Bulldog springs in mine with a 60gr hammer. 20 shots at 85fpe in 25 with a 22.8" barrel. Perfect candidate for a 28" TJ 257 barrel swap. Is already full bore ported in 25. Comes with a 500cc 250bar bottle.
85 fpe is nowhere near enough for .257. You want at least 140 fpe or preferably 160 fpe to get those 257420 & 257388 bullets up to speed. That's doable with Evanix Blizzard and Sniper X2 bottle gun but if Kral only makes 85 fpe in .25 it seems rather low for that. Then there's the issue of barrel support, Bighorn has rather short length of breech that supports the barrel and the rest is free floating. When you attach a 28" or rather a 30+" .257 barrel to that it will whip and quite a lot. In my Blizzard which seems to have longer breech support for the barrel than Bighorn, I had to install a barrel band with no o-rings to get good accuracy. And as for good accuracy I'm referring to sub 1" groups at 100 yards. If I was to build a .257 from scratch I'd seriously consider Kral as a starting point but it really needs to make more than 85 fpe in .25 to make it feasible. If there's not enough hammer strike you can always use a 2mm valve stem to make more valve dwell.
85fpe is with a full 20 shots. It will do a lot more power wise but at the expense of shotcount. There's plenty of hammer strike. I'm no where near max potential at 85 fpe. That's with a single hammer spring and the adjuster almost all the way out. As far as barrel goes there is no issue. Just make a new fitting at the rear of the shroud towards the breech longer to accommodate the extra barrel length and it will remain a tensioned barrel just like the factory barrel is.
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The Bighorn might fill the gap in the unavailability of a bottle Evanix. Allows for a slightly heavier hammer and a lot more spring due the lengthened action. I'm running Benjamin Bulldog springs in mine with a 60gr hammer. 20 shots at 85fpe in 25 with a 22.8" barrel. Perfect candidate for a 28" TJ 257 barrel swap. Is already full bore ported in 25. Comes with a 500cc 250bar bottle.
85 fpe is nowhere near enough for .257. You want at least 140 fpe or preferably 160 fpe to get those 257420 & 257388 bullets up to speed. That's doable with Evanix Blizzard and Sniper X2 bottle gun but if Kral only makes 85 fpe in .25 it seems rather low for that. Then there's the issue of barrel support, Bighorn has rather short length of breech that supports the barrel and the rest is free floating. When you attach a 28" or rather a 30+" .257 barrel to that it will whip and quite a lot. In my Blizzard which seems to have longer breech support for the barrel than Bighorn, I had to install a barrel band with no o-rings to get good accuracy. And as for good accuracy I'm referring to sub 1" groups at 100 yards. If I was to build a .257 from scratch I'd seriously consider Kral as a starting point but it really needs to make more than 85 fpe in .25 to make it feasible. If there's not enough hammer strike you can always use a 2mm valve stem to make more valve dwell.
85fpe is with a full 20 shots. It will do a lot more power wise but at the expense of shotcount. There's plenty of hammer strike. I'm no where near max potential at 85 fpe. That's with a single hammer spring and the adjuster almost all the way out. As far as barrel goes there is no issue. Just make a new fitting at the rear of the shroud towards the breech longer to accommodate the extra barrel length and it will remain a tensioned barrel just like the factory barrel is.
Ah, OK - I thought it was a max power tune. .257 is a few shots gun only if you shoot those heavy bullets at high speed, that's why most people shoot them tethered. Evanix do not tension their barrels, what's the Kral tensioned setup like and could you do it for a TJ liner?
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The Bighorn might fill the gap in the unavailability of a bottle Evanix. Allows for a slightly heavier hammer and a lot more spring due the lengthened action. I'm running Benjamin Bulldog springs in mine with a 60gr hammer. 20 shots at 85fpe in 25 with a 22.8" barrel. Perfect candidate for a 28" TJ 257 barrel swap. Is already full bore ported in 25. Comes with a 500cc 250bar bottle.
85 fpe is nowhere near enough for .257. You want at least 140 fpe or preferably 160 fpe to get those 257420 & 257388 bullets up to speed. That's doable with Evanix Blizzard and Sniper X2 bottle gun but if Kral only makes 85 fpe in .25 it seems rather low for that. Then there's the issue of barrel support, Bighorn has rather short length of breech that supports the barrel and the rest is free floating. When you attach a 28" or rather a 30+" .257 barrel to that it will whip and quite a lot. In my Blizzard which seems to have longer breech support for the barrel than Bighorn, I had to install a barrel band with no o-rings to get good accuracy. And as for good accuracy I'm referring to sub 1" groups at 100 yards. If I was to build a .257 from scratch I'd seriously consider Kral as a starting point but it really needs to make more than 85 fpe in .25 to make it feasible. If there's not enough hammer strike you can always use a 2mm valve stem to make more valve dwell.
85fpe is with a full 20 shots. It will do a lot more power wise but at the expense of shotcount. There's plenty of hammer strike. I'm no where near max potential at 85 fpe. That's with a single hammer spring and the adjuster almost all the way out. As far as barrel goes there is no issue. Just make a new fitting at the rear of the shroud towards the breech longer to accommodate the extra barrel length and it will remain a tensioned barrel just like the factory barrel is.
Ah, OK - I thought it was a max power tune. .257 is a few shots gun only if you shoot those heavy bullets at high speed, that's why most people shoot them tethered. Evanix do not tension their barrels, what's the Kral tensioned setup like and could you do it for a TJ liner?
The shroud is a 3 piece kit. You would just need to remake the section that buts up to the action 5.2" longer to accommodate the extra 5.2" of barrel if you go 28". The muzzle will just need to be threaded 1/2-20 to match the factory barrel and it screws together tensioning the barrel.