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PostPosted: Fri Aug 24, 2018 11:46 pm 
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Location: Southern Gulf Islands, Beautiful British Columbia, Canada
Hello All!

I was doing some FT today with several of my air rifles, experimenting with different pellet weights (all JSB Exact) and noted each time I moved up in weight, the trajectory of my pellet did not change vertically, but scewed instead horizontally to the left instead, and by a substantial degree (half an inch at 35 yards)! Grouping and accuracy more or less remained the same on anything above 14.35 go on my higher powered rifles. I expected the pellets impact point to change vertically, so was quite surprised by the results. Not being an expert in aerodynamics, not sure what’s actually happening here. Suspect it’s the spin imparted on the pellet, but I defer to those more learned than I here on the forum for an intelligent answer.

Cheers!

Avianmanor

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PostPosted: Sat Aug 25, 2018 7:02 am 
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Location: Bowmanville, Ontario
I’ve noticed the same with my springer .22 Benjamin. Any different pellet than what I’ve zeroed in is high, but always right. The other thing that’s odd to me is no matter the weight, heavier or lighter they’re always high...

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PostPosted: Sat Aug 25, 2018 7:55 am 
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If gravity were the only physical force at work, then the only change in POI would be vertical when using heavier or lighter projectiles. Different projectiles, be they pellets or bullets, can respond differently in the same bore. Furthermore, spin drift, is a factor to consider. Forum member rsterne explains it very well:

Spin Drift, is a gyroscopic interaction with the air the bullet is flying through, and requires no wind to take place.... A spin stabilized projectile will drift sideways even with no crosswind present.... The combination of spin and gravity cause the bullet to yaw slighty, and hence skid sideways at an angle, called the "yaw of repose", which causes a lift force to the side the nose is pointing in (makes perfect sense).... A right hand twist (clockwise from the rear) will cause a drift to the right, and a left hand twist to the left.... Factors that can influence spin drift include; bullet length, spin rate, range, time of flight, height of trajectory, and atmospheric density.... increasing any of those increases the amount of drift.... https://www.gatewaytoairguns.org/GTA/in ... ic=94127.0


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PostPosted: Sat Aug 25, 2018 10:45 am 
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Location: Coalmont BC
The effect you are seeing with different pellets not doing what you expect is likely barrel harmonics.... All barrels vibrate, and the direction the muzzle is pointing at the instant the pellet leaves the muzzle governs the entire flight.... If a heavier pellet (which takes longer to reach the muzzle) is hitting left, the barrel is probably high and to the left when that heavier pellet exits, compared to the lighter one, which arrives at the muzzle earlier in the vibration cycle.... The natural drop of the slower projectile is being cancelled out by the barrel pointing higher at the instant of exit, but the left remains.... An offset of 1/2" at 35 yards is only 0.0095" at the muzzle for a 24" barrel.... about twice the thickness of a human hair....

As I mentioned in that previous post linked above.... spin drift can also be part of the cause.... A heavier pellet is spinning slower because of the lower velocity, and is likely also a different length.... These factors could cause it to yaw differently just as it exits the muzzle.... and hence change POI in an unpredictable way.... You nearly always have to resight when changing pellets, so get used to it.... :wink:

Bob

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PostPosted: Sat Aug 25, 2018 11:52 am 
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Because I've made so many changes, I've always just kept the scope zeroed to the baracudas. When I change pellets it's usually for testing. And I just either holdover or just aim at the bull and the pellet hits where it hits.

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PostPosted: Sat Aug 25, 2018 1:47 pm 
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Location: Winnipeg, Canada
Good question, and even better discussion.

The bbl harmonics thing makes the most sense to me~ especially since airguns generally have smaller diameter bbls to keep weight manageable.

It would be interesting to see a test where velocities for different weight pellets is kept close to uniform. An adjustable gun would make this a bit easier.

-D.S.

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PostPosted: Sat Aug 25, 2018 3:38 pm 
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Location: Southern Gulf Islands, Beautiful British Columbia, Canada
Thank you Gents,

for responding so promptly to my post regarding changing pellet trajectory vs projectile weight - very interesting indeed! Since it would appear that velocity is a factor in both horizontal and vertical flight, said information now gives pause for thought and credence to the concept of regulated Air Guns being more accurate than unregulated, regulated producing tighter and more consistent shot groups vertically and horizontally, than unregulated -this has been the case within my collection of air guns. Great to know this is normal aerodynamic behavior and not some issue with firearm or scope.

Cheers!

Avianmanor

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PostPosted: Wed Aug 29, 2018 8:48 pm 
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Location: United States
Are you saying they went 1/2" to the left on all the guns you tested? And am I to assume the heavier the pellet the further left it went? I could see one gun doing it, like rsterne said, but all of them having harmonics do the same thing makes me think the wind picked up, or gremlins. Gremlins meaning I don't have a better explanation. The energy sources that cause harmonics aren't very strong on a pcp, but you also likely have a long thin barrel w/ no support? I mention bc I'm in the US and we have shrouds which offer support, so maybe you could put a non-functional shroud on it? Or do you already have that? Either way I'm picturing adding rubber discs/doughnuts spaced inside the shroud every few inches to support and help dampen any barrel movement. Or maybe surgical tubing; lube it up and coil it inside the shroud end to end so it's a snug yet very even fit on the entire barrel. If nothing else harmonics would at least change so you can see if that's the cause. Or add some weight to the barrel, like zip-tie some steel rods or whatever to it to see if things change. I dunno.... Post if you figure anything out....
Some vertical thoughts: Pcp are better at maintaining velocity w/ heavier pells than say a firearm or springer which both lose power as projectile wt goes up. Pcp gain power so velocity drop isn't as bad. Plus heavies have better BC so vertical changes at range would be small to zero, and even better at some point down range.
Another explanation for unexpected vertical poi could be scope height and the range you have it zeroed to. I spoze you could plug them into Chairgun and see what do?


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