Twist rate
- Thread starter Kitplanenut
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I can't answer that question but that makes me wonder. Does different speeds make more or less twist on the pellet? On a traditional rifled berrel it seems less likely to cause more or less pellet twist regardless of speed because the pellet starts out in the rifling. But on a smooth twist the pellet accelerates then hits the twist. I feel like at higher speeds that pellet will be sent out with more twist.
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The faster a projectile passes through a rifled barrel the higher the rotational velocity and generally the greater the stability.
Thurmond
Thurmond
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Let me explain what I was trying to figure out. On my Impact when I changed barrels from sighted in 25 to the 22 there was about a 1 MOA drop at 50 yards. I can understand that. But there was also about a 1 MOA move to the left. That I couldn't understand. Then I remember reading that twist rate could move POI horizontally. Was wondering if that was causing it. Also there was a difference in the horizontal movement between the 15 and 18 grain pellet. That got me to wondering about the 25 cal, so put that barrel back on. POI was the same as when I took it off, but there was a difference in horizontal movement between the 25 and 33 grain. Now, this horizontal movement is just mice nuts, but enough to notice and got me wondering why. I'm sure someone has a logical explanation. And before someone starts putting it down, I'm completly satisfied with the Impact.
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To a point. I know in powder burners using jacketed bullets, a fast twist rate on a light round will destabilize and tear it apart. You also must remember that pellets are primarily drag stabilized and rotation only assists stability. As far as I know there are no studies that determine optimal twist for pellets. I do know that Harry has done screen testing for smooth twist and traditionally rifled barrels. I don't think Harry's data supported any conclusion of which was better.
I have not found the actual twist that is applied to the Smooth twist barrels though.
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You sighted with 25 then switched to the 22 and got a half inch drop down and left with no adjustment to power settings? That seems backwards to me. Was there any wind? Which direction? Which pellet moved the most? I'm guessing the light pellet moved more.
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Bill, that does seem backwards. But there was a power adjustment. When I first shot with no adjustment accuracy was in the tank. Had to dial back to about a 2 or 3 to get accurate. At that point POI was low left. And you are correct, the lighter pellet moved horizontally the most. There was no wind. I'm just beginning to get a feel for all the gun adjustments
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Kit, that adds another variable to figure out. So, you got one of those mythical dead calm days to shoot. Pellets can feel a breeze that we can't, if long grass is swaying or leaves wiggling there is enough to move a pellet. Different pellets, whether design, weights or caliber will perform differently in the same barrel. When I do pellet testing almost everything I shoot from the same gun ends up grouping on a different part of the target. I do not have a gun that can change barrels so I can't test this for myself. The power adjustment to get the 22 accurate seems to account for the drop and the pellet weight difference would account for the horizontal shift, with a light breeze. Are you able to Chrony your shots? I'm guessing the power adjustment dropped the 22 velocity down to or likely below the 25 velocity and since 22 pellets are generally lighter than 25 pellets you see more horizontal movement. Lower velocity allows wind to deflect the pellet more. Or your 25 and 22 barrels aren't indexing exactly the same so there is a horizontal shift between them. More data would help you figure out what is going on. Also, from a SmoothTwist, lower velocities seem to produce a faster effective twist rate than higher velocities due to the "skidding" effect through the rifling.
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Tom Gaylord did a fairly lengthy test on the effect of various twist rates on pellet accuracy. Look it up on his PA blog.
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I to am confused I don't hav 25 cal but I can zero my 22 cal at 30 yards and to get the best groups my POI at 50 yards is a inch to the left to get it lined up vertically with my zero I'm getting about one inch groups at 50 yards
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Interesting point about that... a bullet can be torn apart by centrifugal force alone, even if it's perfectly stable. A typical .223 Remington bullet leaves the muzzle at somewhere around 3,000 FPS. From a 1:8 barrel, it's rotating 4,500 times per second, or 270,000 RPM. The centrifugal force on the jacket is over 200,000g.
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bill. we have been getting some beautiful dead calm mornings. Have been taking all my airguns out and checking 0 on the scopes. I hear you on the breeze. I've decided to give up on all the different pellets. Even though accuracy on some are a toss up. Like the Wildcat shooting the 15 and 18 grain. Accuracy is the same on both. Same with the 25 and 33 grain on the 25's. I'm going to go with the heaviest pellet and forget the rest. These airguns are getting to complicated. Need to be packing a journal.
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I do this. I document almost everything with my guns. I even have excel workbooks that track my tune settings and results with different pellets.
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No 2 barrels are the same. I would bet my money that no 2 Impact barrels, even if they are in the same calibre will have the exact same poi at 50 yards.
All ST barrels are hand straightened, and even the smallest deviation will have a significant change of poi at 50m/yrd.
Actually, I find a 1 moa difference at 50yrd in between 2 different barrels, and even of 2 different calibres pretty impressive
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Maybe something with how the barrel lock down screw contacts one barrel more than the other,just a though on the sideways poi shift
Rgds steve
Rgds steve
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