Air Stripper / Shroud Tuning Experiment with 50.15gr JSBs

[mtnGhost]

Preface: I made a crazy discovery today regarding the sensitivity of 50.15gr JSB pellets in relation to air strippers / moderators. This post might help someone out who's been determined, yet unsuccessful in getting them to shoot well. I've seen many airgunners dismiss them as incompatible with their barrels on this forum and elsewhere, but the results from my experiment today fascinated me and I felt like sharing it with this community.

I ran this little experiment today with my .30 caliber JSAR Raptor. One of my main side projects lately has been around machining and testing different barrels for this rifle, and trying to fabricate "perfect" shrouds for each one. My current focus has been this 20" TJ, which I previously setup in 26" and then 24" in length. It shot the 50.15gr JSBs very well, and my math told me that I could get away with shortening it even further.

The other day, I rummaged through some of my spare parts and found several FX shrouds, knowing that they'd fit this barrel's outer diameter along with the 1/2-20 thread that I put on the muzzle end. I went a little long when I turned this thread, thinking that I would re-use the JSAR air stripper piece that tensions down a shroud assembly - ultimately stiffening the barrel. This has become a feature that I REALLY prefer with this Raptor, as the harmonics are awful without the tensioned shroud (and thus groups are )!



So, I wind up matching up one of my FX shrouds that has a built in air stripper from God only knows what rifle / barrel kit that it came from. Whatever it was had to have been a .25 cal, so I took the air stripper out of the tube, and I bored out the muzzle with my lathe to a size sightly over my barrel's .300" groove diameter:

The FX shroud installed on my rifle didn't provide a similar tensioning / compression against the rifle's block like the factory setup, so I needed to make a bushing. I also started thinking about muzzle's orientation in regards to the milled air stripper section (knowing that the exposed threads on the FX barrel kits are much shorter than my generous length thread). I calculated the lengths for a few bushings that would allow me to orientate the muzzle crown in 3 different positions in the air stripper:

(Notice how the barrel is recessed in the part)

So I headed out and setup my target a hair over 50 meters. The wind was blowing like mad, but it was pretty consistent from my 10 o'clock, and I was shooting the 50gr around 870FPS, which worked well with this barrel in its previous lives (lengths).

I started off with the bushing that allowed me to screw down the shroud to the complete length of my muzzle threading. My testing methodology:

1. Start off aggressive, and end up as close to the threaded depth that FX barrels wind up at
2. If my group was tight, I would keep shooting; if not, I would adjust the shroud tension and retest
3. Test with and without my DonnyFL Tatsu moderator on the end with each bushing and at each torque 
4. Test with 44.75gr JSBs with the various configurations
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   Ok, so what exactly did I discover?
   
   1. These pellets were extraordinarily picky with the shroud orientation!!! POI shifting was also OFF THE CHARTS!!! It was nuts.
   2. The Tatsu decreased my velocity (consistently), around -20FPS
   3. The deeper muzzle / shroud threading caused severe accuracy problems
   4. The shallower threading allowed me to shoot the 50gr exceptionally well!!!
   5. The accuracy of the 44.75's were not affected by thread depth (not nearly as perceivable at least)
      [/LIST=1]
      
      Now it was nasty out today, and I ran out of splatterburst targets towards the end, but I did save the target to a point that demonstrates the difference between the incompatible shroud tune and the harmonious tune (circled in red). 
      
      
      
      
      The top left circle was 5 shots. The other one is two (9) shot magazines minus two pellets that jammed slightly, which I dumped off to the side of the target. 
      
      Once I found the right shroud tune, my biggest fear was repeatability. I was able to remove the shroud, reinstall it, and find the tune with my zero again and it shot just as well. I suppose my next test will be on a warmer / drier day, but I don't think the weather has anything to do with it. 
      
      The bottom line, is these pellets are VERY sensitive to the air stripper and moderators!!!! I live these pellets though, they buck wind very well for me (just as, if not better than Monster RDs)!
      
      EDIT: FWIW, I'm running a 45 degree chamfer on my muzzle crown

 

[biohazardman]

That was a bit of werq you went through. Guess that just goes to show the effort that can go into tuning these guns so they shoot their best. Thanks for taking the time to post your werq.

 

[mtnGhost]

That was a bit of werq you went through. Guess that just goes to show the effort that can go into tuning these guns so they shoot their best. Thanks for taking the time to post your werq.

Yeah man, this is one of those little details that can be taken for granted.

I did some testing similar to this when I made my FX barrel adapters for my Taipan. I had to nail the length just right to get the threaded piece to cooperate with the Veteran's factory moderator (the liners are recessed slightly in those stainless FX threaded adapters, and the exit gasses can work against the projectile's stability as enters the expansion chamber). All you can do is exercise trial and error until it's perfect.

That said, you have wonder how many times pellets or slugs have been written off as incompatible with certain barrels / rifles, when all that's required is +/- a millimeter or so of threading.

 

[Hoople]

I have often wondered why more work has not been done with the 50gr JSBs. They clearly have the best BC of any of their other pellets, and would be the ultimate for 100yd BR and long distance pellet shooting. Good to see someone with the resources and talents to finally do that. Thanks

 

[jwrabbit123]

I kind of noticed same thing with my R-5 meaning it shot well with 44 gr but the 50g would wobble , I got them to run true by playing with the crown and the nut barrel tension , but for some reason if you removed the barrel it took alot of fiddling to get poi and it to shoot the 50 gr well , so I stopped shooting them , I never spent much time trying to get the poi to be identical with them but they were sure finicky , with airflow 

did you notice the larger groups if the pellets were wobbling a bit the ones with bigger groups?

 

[Motorhead]

That is some weird poop right there ! ..... odd because the portion going onto barrels threads is not really an "Air Stripper" but merely a tensioning cap with VENT to allow pressure / air volume into the shroud space.

My guess is that the root of the actual goings on is that of SQUARENESS and how true the compression of tube is maintained. * Screwed on just a little what there is side for side radial motion available. It can tilt some so it squares up with tube better. Where screwed on further the overlap of threads now increasingly hinders any side to side / tilt and when snugged down may indeed be creating non uniform pressure / contact upon shroud tubes end.



ALSO .. make sure shroud tube ends are perfectly square !



Having played with tensioned barrels quite a lot in recent years have found this happening a couple times.





** What i would do ... Take out barrel and place in LATHE, screw on the "Stripper" just shy of threads bottoming out, place a live center in tail stock and put some tension on it.

With a dial indicator index off the area shroud tube is in contact and see how consistent the run out is ? * Make a felt pen mark to know where started. Then rotate the "stripper" on the barrel threads 1/3 turn ( 120* ) and measure again, then 120* again.

IF YOU SEE CHANGES it is telling the barrels threads or "Stripper" threads are cut crooked. And this will support my thoughts of further it's threaded on, less it can self correct for being out of square and worse it shoots.





Just sharing thoughts ...



Scott S

 

[mtnGhost]

Hey Scott, I like how you think. I check the already on my V-block rollers already with my dial test indicator after I bored out the part. I use a long piece of flat steel for my V-blocks, and then I use my super magnetic base with my 1/1000 mm DTI. Concentricity doesn't seem to be the issue with any of the components, but I would have suspected them if they were

The points that I find interesting:

• Accuracy of the 44.75gr were not noticeably affected
• I have also seen this type of problem with the FX barrels going into the Taipan Veteran factory moderators (similar results with 3 different barrel kits and two different moderators)
• Rifle is accurate unshrouded at low reg pressure (harmonic problems seem to manifest over 2000PSI in this rifle)

 

[mtnGhost]

I kind of noticed same thing with my R-5 meaning it shot well with 44 gr but the 50g would wobble , I got them to run true by playing with the crown and the nut barrel tension , but for some reason if you removed the barrel it took alot of fiddling to get poi and it to shoot the 50 gr well , so I stopped shooting them , I never spent much time trying to get the poi to be identical with them but they were sure finicky , with airflow 

did you notice the larger groups if the pellets were wobbling a bit the ones with bigger groups?

That's what I forgot to add to my original post! There does appear to be some slight keyholes in my target. My R5 eventually shot them well (literally perfect when the barrel was clean), but only after I sorted by head + skirt size and weight. You might also be in to something, because when the barrel nut wasn't tensioned to a specific torque value, they sucked!

I have often wondered why more work has not been done with the 50gr JSBs. They clearly have the best BC of any of their other pellets, and would be the ultimate for 100yd BR and long distance pellet shooting. Good to see someone with the resources and talents to finally do that. Thanks

I'm in 100% agreement. I would much rather hunt with these than their closest competitor (IMO), JSB Monster (Redesigned), given how they buck the wind. BC in itself isn't the sole data point of real world accuracy, there's other factors and all - but my K.I.S.S. principal has been the heavier the better.

I was chatting with another airgunner whom I greatly respect for his testing and research about both pellets. I was amazed when he said that he determined that both were 150y pellets, since I had also concluded exactly that during my testing. What is interesting to me, both of these pellets seem to be more finicky than most others.