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Author Topic: velocity/accuracy  (Read 191 times)

EdLena

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velocity/accuracy
« on: August 07, 2017, 05:43:37 AM »
Hi Bob,

I registered for the EBR (sportsman class) and my AAs510 .25 cal I believe, is not good enough yet. I've done a lot of modifying (all based on your posts btw) including porting to 80% of bore, an antibounce hammer and a huma regulator with a 34cc plenum volume which you helped me choose. Also a carbon fiber sleeve, secured to the receiver over the LW barrel and I'm working on getting an air stripper aligned and adjusted. Using jsb 34g pellets it's capable of 945 fps average with a <1% ES for 14 shots. I'm in the process of lowering the velocity to gain shots per fill so I won't have to refill more than once per secession and to improve accuracy, I hope. Ted Bier claims he had a 1 MOA 10 shot group at 100 yards shooting the same pellet at 830 fps. He says he has no flyers at that velocity but he is using a smooth twist barrel. I currently have mine shooting at 880 fps with the regulator and hammer force adjusted, again by your method, but there's no real improvement in the number of flyers. Right now it will occasionally shoot 1.25" groups at 100 yards with a clean barrel and a light wind.
My main question is how low of a velocity do you think I should go to hopefully reduce the group size and the number of flyers or would it even matter?

Thanks,
Ed.
 


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rsterne

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Re: velocity/accuracy
« Reply #1 on: August 07, 2017, 08:23:00 AM »
I don't have enough experience to tell you.... If you are not experiencing spiraling, and are getting consistent groups at 100 yards, I don't know if reducing the velocity is the answer or not.... I suspect that the 830 fps Ted mentioned is good for HIS gun because that is where harmonic barrel vibrations match the velocity.... Before reducing the velocity, I would try tuning the harmonics of the barrel.... The air stripper you are installing may well do that.... If not, then make or obtain a harmonic barrel tuner that will install on the same threads for the air stripper, and try that.... Personally I wouldn't go below 880 fps unless you have exhausted all other options....

The last stripper I played with at 100 yards had two sweet spots, one at just under 1/4" from the muzzle for the cone, the other at about 1.5 calibers (3/8").... but the larger gap seemed to have a slightly wider range of adjustment, so I settled on that.... What I didn't get to try was to see if a different velocity would have a different setting, but I suspect that would be the case.... One thing I haven't tried is threading the outside of the air stripper for an adjustable sleeve, so that the gap and position of the weight can be tuned separately.... ie a harmonic balancer weight on the OD of the stripper, forward of the cone....

Mike of Thomas BR Rifles says that flyers are the result of pellets picking up lead deposits from the previous string of pellets.... ie each pellet deposits a small amount of lead in the barrel, and then one suddenly "cleans" it, and heads off in a different direction as a result.... I know he cleans his barrels a LOT, like maybe every 20-30 shots.... but I am nowhere near that level of shooter, and cannot advise you about that.... However, I wouldn't bet against his opinion....

The other thing that could be happening is that you are not experiencing flyers, but it could be the natural dispersion of the shots, the more shots you shoot.... Read this article, it may you to help understand that....  http://www.the-long-family.com/group_size_analysis.htm .... The basic idea is that the more shots you shoot, the larger the group gets, for any given gun.... A rifle that consistently shoots MOA with 10 shot groups is simply a more accurate rifle than one that can only manage that with 5 shot groups, by about 30%.... In fact, 4 shot groups are the truest indication of the MOA ability of the rifle.... A 1 MOA rifle will most commonly shoot 1" 4-shot groups at 100 yards, but if you shoot 10 shot groups, they will typically be 1.5"....

Bob
« Last Edit: August 07, 2017, 08:31:21 AM by rsterne »
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EdLena

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Re: velocity/accuracy
« Reply #2 on: August 08, 2017, 05:18:32 AM »
"One thing I haven't tried is threading the outside of the air stripper for an adjustable sleeve, so that the gap and position of the weight can be tuned separately.... ie a harmonic balancer weight on the OD of the stripper, forward of the cone...."

I actually tried that. Using a hatsan stripper I straightened the cone shaped front and then I tried threading it for a 3/4" lock nut. The tight fitting nylon insert would have kept it from moving on it's own and I could use a wrench to screw it in and out to find the best distance. Unfortunately there wasn't enough meat on the stripper and it collapsed during threading. A simple sleeve could work and it could be held at different distances with a set screw.
I'll try it and get back to you in a few weeks.

I agree that other than a coincidentally favorable barrel tune on Ted's gun there is no reason to drop the velocity below 880fps. Reading your post on velocity and wind drift it is a good speed. Then I read your post http://www.airgunforum.ca/forums/post427562.html#p427562  on the subject and saw you were able to refine the ideal velocity for different weight and BCs of .22 pellets/bullets. Have you ever calculated the best velocity for Bc and wind drift for the jsb .25 34g pellet?

A special thanks Bob for all those great informative posts!
Ed.
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rsterne

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Re: velocity/accuracy
« Reply #3 on: August 08, 2017, 10:13:10 AM »
I haven't done a complete drift vs velocity curve for that pellet.... to do so accurately require a complete Cd vs Velocity curve, as it likely is not a perfect match for the commonly used GA drag model.... At 100 yards with a BC of 0.050, the GA model goes through minimum drift at about 850 fps muzzle velocity.... but it is so flat between 800-900 fps that IMO that favours being closer to 900 fps, if the accuracy is there.... Even 950 fps has only 4% more drift than the optimum....

Bob
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