On Tuning a Stormrider

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I'm tuning my Stormrider II to shoot the Griffin 13 gr slugs.

I am adjusting the location of the front barrel band (which I added to the rifle).

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Observe that the band position was 40, 50, 60 and 70 mm for that target. The 40mm group is stretched out vertically. The 50mm group is round. The 60mm group is stretched out vertically and the 70mm group is stretched out horizontally. I digested that and then shot this target.

1588101904_1066986285ea88310064de0.09017168.jpg


You see here the group with the barrel band at 80mm is again round. So decided to try a group at 65mm to see if I could get a group which was as narrow as the one shot with the band at 60mm and as short as the one shot at 70mm. IOW a round group that was smaller than either of the others. You can see that I did. The other two groups on the target were me just fiddling with the zero.

Then I shot this target to confirm the zero.

1588102070_10451900875ea883b6123011.62440152.jpg


I realize these are not impressive groups, but clearly there are sweet spots for placing that barrel band along the air tube.

Does anyone have experience finding more than one of these nodes or sweet spots on the same rifle. I'd like to further reduce that group if that is achievable. Also interested in other thoughts on tuning this particular rifle.
 
I like your style. Experimentation is the name of the game with optimizing for harmonics. If you’re looking for a good starting point in general, try 4/7 of the distance from the receiver to the muzzle. Next best is 3/7. Again, speaking of generalities relating to harmonic nodes. Experimenting will have the final say. 

Also, if you haven’t checked already, give the barrel a close inspection. Rough leade and crown are a near certainty with a budget rifle like this. Not judging, I have two SPA rifles myself so I’m speaking from experience. Unfortunately, they are also bad about damaging the rifling at the muzzle: 
https://www.gatewaytoairguns.org/GTA/index.php?topic=166821

The good news is they can be remarkably accurate if you deal with those shortcomings.
 
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Yo’s comment made me look back and it sounds like you added a 2nd barrel band. Is that right? In that case, try it at 4/7 the distance from the OEM band to the muzzle.

Let’s say there is 14” from the original barrel band to the muzzle (or the end of the LDC in this case). Try placing the 2nd band at 14 * (4/7) = 8” from the original barrel band. If that puts it past the end of the air tube, try the 3/7 position.
 
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Yo’s comment made me look back and it sounds like you added a 2nd barrel band. Is that right? In that case, try it at 4/7 the distance from the OEM band to the muzzle.

Let’s say there is 14” from the original barrel band to the muzzle (or the end of the LDC in this case). Try placing the 2nd band at 14 * (4/7) = 8” from the original barrel band. If that puts it past the end of the air tube, try the 3/7 position.
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What makes 4/7 special?
 
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Because natural vibration nodes tend to set up at divisions of 2, 3, and 4 of the unsupported length (25%, 33%, 50%, 67%, 75%), and the idea is to avoid these and instead place the barrel band at a probable antinode (location of high displacement). 4/7 or 57% is that probable location.
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Sometimes I just have to be told the obvious =) This I should have understood intuitively. =)
 
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Ive tried barrelband in some different postions, and also 2 barrelbends, I have a couple Diana stormriders, but the main problem with the "harmonics" is that is seems to shoot higher when the pressure gets lower, 200 bar= bullseye, then 180 bar a few mm higher, 160 bar even higher and so on, it has nothing with velocity do to, it seems to be the harmonics vs the pressure holding the valve when hammer hits, its hard to find a sweetspot as there are many setting one can do. 

I didnt put to much time to find out, as each gun has it own way, and its to time consuming , and I dont have high expectation of this gun shooting FX/AA good anyway.

What Ive noticed is, 2 barrel band are better then one, In my case, and I think a regulator should take care of the most problems in my case, but the regulator alone is half the price of the gun, so....

Im about to within a few month as I dont have much time now, I will glue on a carbonfiber-tube in between the receiver and the barrelband, it should stiffen up things quite good.

I think the main problem is the thin barrel. Ive also order som extra barrelbands in plastic from Ebay.

I can post my result when I dig into this again.

Ive done inlet and crown work, polished barrel, making sure the receiver is flat against the tube, and made sure the grubscews are firm onto the barrel. nothing helped much except extra barrelband.



But as other said, check inlet for burrs, check the crown with cottonswab for burrs, make sure the receiver is flush on the tube, and feel the grubscrews, they are poor material.

Maybee this above can help you some? It didnt help me much, but I have high hopes in the carbonfibertube on barrel :)


 
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Yeah it’s nice that carbon fiber tubing is so inexpensive nowadays that there’s not so much hand-wringing to just give it a shot. Unfortunately there’s not enough meat in the receiver to accept the larger diameter but it may still provide some benefit to simply sleeve the exposed portion.

I’m curious, where is the good part of the bell curve? For example, you mention that the POI is continuing to climb as the pressure declines to 160 bar (~2300psi). This review on AGN says the optimal fill pressure is just a bit above that so what you’re seeing sounds normal. But manufacturing tolerances being what they are, who knows!

https://www.airgunnation.com/topic/best-budget-2000-psi-rifle/
 
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it was some time ago, I dont remember everything exactly, but yes, speed starts low, then rises (bell curve), but after that it riser more and more, this is where my harmonics comes into play, I will get to the bottom of this, and I hope my carbonfibertube over barrel + extra barrelband will cure most of that problem. The difference in speed from 200 bar to lets say 150 bar shouldnt rise the POI so much as mine did, so for sure there is some harmonics problems.

I will also look into how to secure the receiver to the tube extra.

I also have to order new grubscrews, as the ones that came with the gun are soft.




 
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After all of that I shot these additional targets:

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I then measure all the targets that I shot. I put them in a spread sheet and plotted the maximum diameter, maximum width, maximum height, and mean radius against the distance the front edge of the band was from the end of the air cylinder.

I got a graph which really says it all:

1588532527_13150394935eaf152f7e9591.86837099.jpg


This graph really explains what is going on with the tip of the barrel while you are trying to find that magic spot. It even shows what the best group one should get out of the specific projectile being tested. So this one, the 13 gr griffon, will give me groups around 17mm at 25 yards. Just look where the width and height cross. So about the best I should expect without stiffening my barrel somehow is about dime sized. That will do for now.

After the above, I went out and shot these groups with the barrel band set at 63mm.

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The three on the right are 5 shot groups with the Griffin 13 gr. They averaged 13.5mm overall with an average width of 11.6mm and an average height of 10.4mm. So the prediction holds pretty tight to the graph. I then shot 20 shots of 10.5 Crossman Premier directly from the tin. I didn't even look at the skirts. The first sixteen went into less than 24mm and of those shots 5 thru 16 made the dime sized cluster. So good to go. It looks like a charge of 180 bar would give me one magazine well under a dime at 20.4 FPE.
 
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I don't have a picture, but will take one tomorrow. My Stormrider is 100% unmodified. I see much better grouping than this.
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I'll be happy to see that. Mine has had a lot done to it. I redid the crown and the leade as they were rough. I removed the baffles from the moderator as they seemed to be clipping. I replaced the plastic stock with a Gen I wood stock because this old man needs a bit more weight in a rifle. I also printed a part on the 3d printer to fill in the gauge hole in that stock. I installed the regulator from Altaros that is sold under the Diana label. It is set to 132.5 BAR. This is about the best I have had the gun shooting at this point but some of the other fellows are shooting better groups for sure. It is shooting 13 gr Griffin slugs at 841 fps for about 20.4 FPE. It will do that for 16 shots before it drops off regulator real hard and throws shots low. The whole history of the rifle is here: https://www.airgunnation.com/topic/stormrider-experiences/

13.5 mm average diameter for 3 five shot groups, pretty much any PCP should be able to do that so you've seen mine 😉 show me yours. 😃
 
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As promised. The first 3 shots of the first groupI shot out of the magazine. I don't know what pellets my son had in there but obviously they are wrong. I didn't reshoot it because that wouldn't have been right. The second 2 groups are JSB Jumbo's. These are the first 15 shots I fired. 3 targets, 25 yards, 5 shot strings.
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Very nice, very nice.  I get pretty good results with JSB Heavies and H&N FTTs too. Slugs seem to be a bit more problematic. I do have it shooting under a dime with the Griffins.

More on those pellets here: https://www.airgunnation.com/topic/stormrider-experiences/page/2/
 
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