Pellet speeds vs lateral pellet drift

[frank320]

With my FX Impact X 25 cal, i am able to shoot the same pellet at various speeds easily. Currently, I have it shooting JSB 25 cal Mk2 at 895 at Max setting, to as low as 725 at Min setting, and everything in between power wheel settings. Here's the weird thing.

Shooting at 30 yards in the basement with no wind, with the zero set perfectly at 895fps at Max power wheel. When set at Min setting shooting 735fps, the pellet lands low and left. The low power Min setting is consistently left by about 5mm. Intermediate low settings are also drifting left by small amounts, but still noticeable. The low is exactly as expected, but left? 

I was hoping to be able to just turn the elevation knob and be able to shoot at different speeds using the same zero, but now i have to turn the elevation and windage turrets to shoot at different speeds? Shouldn't it just land perfectly low in a straight line?

Anyone experience this?

 

[ackuric]

Air rifles still utilize a minimal amount of twist in the rifling which in turn spins the pellet, a huge difference in pellet rotation will result in horizontal change in POI. I notice it as well. Its only dramatic with huge changes in velocity, but its always there to some degree with any change in velocity. FWIW.

 

[ackuric]

Ideally to maintain minimal horizontal change in POI, you'd reduce pellet weight to maintain similar or near similar velocities to the other tunes. I shoot between 680-850 FPS on various pellet weights and my horizontal change in POI is present, its generally only a few clicks off though.

 

[frank320]

Thanks again ackuric. This information will save the ocd in me from trying to fix the "lateral" drift at lower speeds from my higher speed zero. 

They are indeed off by a click or 2 at most.

 

[EdLena]

The same thing happens when you shoot a non regulated airgun and the velocity gradually drops as a result of lower power. As I understand it, when you fire the same gun and pellet with different amounts of power the barrel harmonics change and result in a different POI.

You have a 30 yard long basement! I'm jealous haha.

 

[Bob_O]

EdLena,

It makes more sense to me that the POI shift at close range would be due to harmonics more than anything else.

 

[ackuric]

barrel harmonics are more UP and DOWN opposed to side to side, so Bob_O is wrong. If you understand how projectiles spin, you'll understand they drift as they do such...



For example, for a 33.95 gr pellet traveling at 700 fps to 30 yards, the travel time to target is .132 seconds. For that same pellet traveling 895 fps to 30 yards, the travel time is .104 seconds, a reduction of 20% less time spent traveling...thats translates to much less pellet rotation from muzzle to target...



If you don't think the above amplifies gyroscopic precession when shooting pellets at a lower speed...I've got a physics book or two for ya.


Additionally if you can shoot at 25, 50, 100, and 150 yards even at the same speed and see zero change in horizontal POI EVEN indoors, I'd be IMPRESSED. Very Impressed...

 

[Bob_O]

Since when is a person wrong for agreeing with what another person says? All I did was say it made more sense.....please don't take it personally.

Matt Dubber talked about spin drift needing to be considered at ranges past 100yds. It doesn't make sense to me that the pellet would "drift" at such close distance.

 

[ackuric]

I am not taking anything personally, and neither should you. I am just pointing out that barrel harmonics are more a function of up/down and the resulting change in POI for his conditions are more likely to be due to the HUGE difference in pellet velocity, the spin imparted upon the projectile and the effect of gyroscopic procession...thats all. Matt dubber's discussion on spin drift was probably more in line with the same tune, not changing from nearly 900 fps down to 700's...just saying.

 

[frank320]

You have a 30 yard long basement! I'm jealous haha.

Haha, yeah, lucky in that sense. To boot, I have 2 shooting lanes in my basement, one for 10/15/20 yards, and another dedicated 30 yards shooting lane. And both lanes can be opened at the same time, with shooters being able to walk between both via a safe side lane.  

Between my 8 year old son and I, we must have put at least 30,000 rounds down range my basement alone.

 

[frank320]

So, out of curiosity, I shot our regulated W.A.R Flex 357 all the way from 4000psi down to 1500psi, with reg pressure of 2750psi, shooting a 81gr JSB from 885fps down to 650fps, and noticed the same low(expected) and left drift as velocity dropped over 200fps when shot at 650fps than from its zero at 880fps.

So, conclusion? Above phenomenon i described is quite typical, at least for both of my higher end PCP rifles. In other words, nothing wrong with my setup, tune or my shooting technique...

 

[ackuric]

Correct frank, its just physics and the nature of gyroscopic procession, imagine what its like to calculate that into your round as a sniper shooting 1000m + ....its insane.

 

[ackuric]

FWIW, Hawke Charigun Pro has a Gyro Coeff, but as you can see that changes based on speed...if you find out your coefficient (using the Hawke Gyro Coeff Calculator under toolbox) at one particular speed however, you can implement that into chairgun and it will show you the drift along your start/end range... 



But the trickery is your Gyro Coeff changes with speed, as does your pellet as it heads down range...which I do not believe Hawke accounts for. So I'd expect even more drift than what it calculates personally...unless it does modify the Gyro Coeff as pellet speed drops...(which I doubt) But for our typical range for air guns its quite acceptable IMO...for a sniper round @ 1000m on the other hand...you'd likely be off unless you used a more precise ballistic calculator.



The closer to max range you use to determine your Gyro Coeff the better, as it will then be an average from muzzle to max range...HTH!

 

[Bob_O]

The only gun I shoot at drastically different velocities is my FX Streamline. Using the power wheel for High, Mid and Low power, I get roughly 925fps, 710fps and 500fps.

I've shot extensively with all three levels in my farm pesting activities. With my gun, POI changes in the vertical axis only.

To maintain zero at 30 yds I have to dial Up 2.5 MOA for Mid power and 9 MOA for Low power. I don't have to touch the windage turret.

YMMV.

 

[texagdds]

I’d index the barrel - all barrels have a curve to them. You just want that curve to point up or down. just my two cents. 

 

[ackuric]

Thats amazing that your rifle shoots straight no matter what Bob. For others that experience the effect naturally from physics... here is more information...



The following variables affect the magnitude of gyroscopic drift:

• Projectile or bullet length: longer projectiles experience more gyroscopic drift because they produce more lateral "lift" for a given yaw angle.
• Spin rate: faster spin rates will produce more gyroscopic drift because the nose ends up pointing farther to the side.
• Range, time of flight and trajectory height: gyroscopic drift increases with all of these variables.
• density of the atmosphere: denser air will increase gyroscopic drift.

Projectile RPM = MuzzleVelocity X (12/Twist Rate) * 60

As you see bullet RPM or SPIN is dependent on Muzzle Velocity and WILL change based upon it...



Also the time of flight changes it, which again I proved in the above post increasing 20% at just 30 yards going from 900 to 700 fps...



Thats 2 factors greatly effecting your gyroscopic drift when reducing muzzle velocity greatly from one tune to another 

 

[ackuric]

Also note OP is drifting left when shooting lower? Twist rates are typically to the right...more spin at higher speeds = more right, less spin at lower speeds and higher time of flight = to the left...nothing but external ballistics!

 

[frank320]

Not to beat on a dead horse, but out of curiosity, I enabled the spin-drift calculations on my StrelokPro, played with the numbers using 1:16 twist rate for the barrel and input all the parameters as best as I can. You can see that the higher the FPS, the faster the right hand spin on the pellet/bullet, the more "right" it goes due to the gyroscopic effect, as per mentioned by ackuric. So, it stands to reason that since i zero'ed my Impact at 900fps at 30 yards, if i shot it at a much lower FPS, say 650fps where the pellet's spin is much lower, it would spin drift less to the right and end up low and left - exactly as what i experienced. 

I could even work the math backwards on StrelokPro, and the low and left was exactly spot on for what the StrelokPro predicted I should be hitting. Perfect. I love it when actual experiments match the theory. 

Bob_O - you may not experience this with the FX Streamline, possibly because it is a smooth twist(not X) barrel and the resulting pellet spin forces are much lower. If you have minimal to no twist on your barrel, you will get minimal to no spin drift in perfect conditions.

ackuric - you nailed this one spot on.

 

[flynviking]

I was very skeptical that spin drift caused by a decrease in RPM would have any measurable effect at 30yds. I ran many iterations of twist rate and pellet velocity through StrelokPro and at 30yds it makes virtually no difference to your POI and certainly not enough to account for a 5mm shift in impact.

It is far more likely that some other factor is causing your POI shift than gyroscopic drift at the distance you are shooting and StrelokPro backs up that assessment.

 

[frank320]

It is actually exactly 1/10 of a Mil left at 30 yards due to a difference in pellet speed between shooting at 900fps vs 650fps on a 1:16 twist barrel, JSB 34gr MK2.