The mystery of grouping to the left & right: Two possibilities

by B.B. Pelletier

This post was inspired by Ozark, who wondered why his groups were going to the left and right of the aim point. What he either didn’t say, or I didn’t pay attention to when he said it, was that each group was shot with a different pellet. That’s normal for different ammo, but sometimes a gun will shoot the same pellet to one side or the other, and that’s not right. Today, I want to talk about that.

Why do they do it?
There are two principal reasons for this phenomenon. The first is that the scope axis doesn’t align with the axis of the bore. You took my advice and sighted-in at 20 yards, but when you did you noticed something strange. I told you that if you were sighted to strike an inch below the aim point at 10 yards, you would be approximately dead-on at 20 yards. You were for elevation, but not for windage. Your pellet struck 1/2″ to the left of the aim point. Oh, well, no problem, you thought. You just adjusted the scope until it hit the aim point and figured you were done.

You were done for 20 yards; but when you went out to 30 yards, your pellet was 1/4″ to the left again. It was right on for elevation but not for windage. And, at 40 yards, it’s 3/4″ to the left. What gives?

The solution
What’s happening is your scope is not looking along the same line as your barrel. At distances closer than 20 yards, the pellet will strike to the right of the aim point, besides striking lower. At 20 yards, it’s right on. Between 20 and 30 yards, it will strike right on for elevation; but after 20 yards, it will start moving to the left (windage). The pictures should make it clear.


The scope is looking in a slightly different direction than the bore of the rifle.


This is what it looks like on paper.

Second problem
Believe it or not, some pellets spiral as they travel downrange! On a sunny day at the range when the sun is at your back and the distance is 50 yards or more, you can see the pellet fly downrange through the scope. Many shooters, including me, have seen pellets travel in a spiral path.

I don’t mean the pellet is spinning on its axis, which is caused by the rifling, though that is also what causes it to spiral. I mean that the pellet is traveling in an ever-increasing circular motion, and it circles in the same direction as the rifling. The center of the spiral is not inside the pellet. If the rifling was right-hand twist, the pellet goes downrange spiraling clockwise. If a left-hand twist, counterclockwise. The pellet only does this when it is not stable – including situations of over-stabilization.


Pellet travels in a widening spiral.

Groups from a spiraling pellet will be both to the left and right of the aim point, so it is more difficult to pin down than the scope/barrel alignment problem. It doesn’t just travel left to right or right to left. It goes back and forth several times as it flies downrange. I have no proof, but I think that pellets that do this eventually fly off in some direction and do not continue the spiral. They are unstable throughout their flight. At close range, they are so predictable that you can even zero a rifle and use it. You’ll get tight groups, but they’ll just migrate around the point of aim at every distance except the distance at which you zeroed the rifle.

One last observation. Pellets that spiral are also falling throughout their flight, so despite my illustration showing them climbing higher at a farther distance from the muzzle, the axis of the spiral is on a downward slant. They don’t really rise as I have depicted here.

Those are the reasons for pellets that move side to side. You can test for this by setting up tissue-paper targets aligned with a laser and collect the targets to analyze what the pellets are doing in flight, but all you really need is a sunny day at the range with the sun at your back and about 50 yards of distance. With the right pellets, you’ll see it for yourself.

31 thoughts on “The mystery of grouping to the left & right: Two possibilities

  1. Off topic. I’m wondering if you’ll be reviewing the new Webley Typhoon? The reviews at Review Center are pretty dismal. I’d love to get your take. Thanks.


  2. What would be a good fix for this? For the scope misalignment, perhaps an adjustable scope mount? As for the spiraling, I’ve heard of it with blackpowder. Would a change of ammo help, or is this something that has to do with the rifle itself? Also curious about an [electronic?] range finding scope: have you ever heard of one, other than a mil-dot?


  3. jp

    the really nice rangefinding scopes have a laser built in and it finds the exact range (to the meter) via laser, and displays it on the lens as you look at the object. very nice indeed, though rather expensive. i use a portable 400m rangefinder, and a seperate scope, thus saving over $1000



  4. JP,

    Aligning the scope with the bore through the use of an adjustable mount will solve the off-axis problem. And you are correct that changing pellets is the solution to the spiralling problem. That problem is caused by instability of the projectile – not the rifle that fired it.

    B.B.



  5. Bullets Vs Pellets

    How come an airgun doesnt fire a bullet accurately, and yet a firearm never chooses to use a pellet shaped bullet? At what point does one become better suited as a projectile? I assume its speed? but some firearms actually propell their bullet slower than some airguns can send a pellet?


  6. BB,

    I have a theory regarding the spiraling flight path of pellets, and being a Flight Instructor, associate it with my knowledge of aerodynamics.

    Air is a fluid, and any solid object that passes through it is capable of producing mechanical forces. In aviation we call these forces lift and drag. The faster an object passes through a fluid, the more lift and drag is produced. Because the pellet is spinning on its longitudinal axis, the forces that it produces are in all directions perpendicular to the flight path and to its longitudinal axis, with MOST of the drag acting opposite the direction of travel.

    So why doesn’t the pellet fly true? Any imperfection in the trailing edge of the pellet (its skirt), OR any change of its TRUE center of gravity vs. its PHYSICAL center of gravity, will cause the forces to be asymetrical. The shape of the trailing edge of an object determines its drag. The trailing edge of an airplane’s wing tapers to a thin knife edge, and therefore produces very little drag. Pellets do NOT have an aerodynamic trailing edge. The shape or angle of the skirt to the true longitudinal axis introduces an angle of attack to its flight path, and therefore is capable of producing lift. Also if the center of gravity is the slightest bit off, the pellet will wobble and change its angle to the flight path. As this angle changes, so do the forces acting on it, and the pellet will destabilize and begin to spiral around its intended flight path. The fact that its center of gravity is forward of its physical center, keeps the pellet from destabilizing to the point of tumbling.

    When I first noticed my point of impact changing left or right at different distances, I thought of scope cant, or not having the center of the scope and the barrel aligned vertically. But after witnessing for myself a spiraling pellet on a sunny day, I have discounted my original theory.

    Michael in Florida


  7. Bullets vs pellets,

    Firearms and airguns use two different methods of stabilizing their projectiles. Firearms spin-stabilize bullets with rifling. Pellet guns use a high-drag projectile called a diabolo pellet to stabilize itself. Even though they are rifled, the twist rate is too slow to stabilize a solid projectile (a bullet) at the velocities airguns can generate.

    Arrows fired from longbows also spin in flight, but it is the drag from their fletching that keeps them pointed straight.

    And some firearms DO USE pellet-shaped projectiles for stabilization. The French Balle Blondeau shotgun slug of the 1960s was a remarkable leap forward in shotgun slug accuracy. Big Bore maker Gary Barnes copied the shape for some of his projectiles and has gotten great accuracy out to 200 yards.

    I could continue, because what you have asked really requires a book to answer, but this is the short version.

    B.B.



  8. Bullets vs pellets,

    thanks, that makes sense now. So the Daystate airrifle you refered to in an earlier post that shoots bullets, only remains accurate because it uses a firearm barrel with firearm rifleing?

    Its an interesting subject :)



  9. BB,

    I found this post which describes exactly what has happend to me on three occassions since receiving my 850 Air Magnum in May of this year.

    “…I recently received my new RWS Hammerli 850 AirMagnum .22 and took it out shooting. The first day it was used at 4000′ elevation and 90 degrees temp. at the inlaws ranch. I managed to shoot about 10 or 15 rounds before the gun seemed to lose all power. The pellets travelled no more than 30 feet and the report was like a “dud” at the muzzle. In fact I heard the spring mechanism operating more loudly than the gas expelling. At first I thought the airsource cartridge must have leaked and decided to replace it. I used some teflon tape on the threads just in case it might help stop a leak if there.

    The new cartridge performed exactly the same way. There was no power from shot #1. The pellets barely left the gun under power. The next day, at home I took it out in the back yard (300′ elevation and 95 degrees temp.). The gun began to work flawlessly – again for about 10-15 shots and then dropped off to nearly zero power. I took the gun inside – to let it “cool” off, thinking the heat may be the culprit here (but not understanding how or why). A while later, I tried the gun again and it worked fine – once again, only for a few shots then just as before lost all power with a relatively full cartridge…”

    I live in central Florida, so obviously I’m not at 4000 feet elevation, and my model is a .177, but the symptoms were the same. I found this response on the same website

    “..Sounds like valve lock. Actually, at temperatures above 89F, CO2 goes “supercritical” and all the liquid is converted to gas no matter how high the pressure rises.
    All airgun valves have a maximum pressure above which the hammer can no longer overcome the force holding the valve closed.
    Sounds like you’ve found that limit for your Diana.”

    Is there a threshold of heat and humidity which would cause the symptoms listed above?

    Michael in Florida


  10. Michael,

    I have never experienced this or even heard of it. When I tested the .177 850 AirMagnum, the day was 95 degrees at 600 feet above sea level.

    CO2 follows a pressure curve as the temperature changes, but I don’t know what that comment about supercriticality refers to. I have shot CO2 guns on 100-degree days without a problem.

    However, because you and someone else have the same problem with the same model airgun there is definately something going on.

    I agree that it sounds like valve lock, but I’m baffled as to why you would have it.

    Perhaps humidity does also play a part in this. I don’t have exoerience with central Florida, but my wife tells me the humidity is always high. Where I am 50 percent is considered high.

    B.B.


  11. We would welcome 50% humidity here. I don’t recall the rifle losing power earlier in the summer, so I’m also guessing that the humidity may have something to do with it. I havent had problems with any other CO2 rifle, only the 850. As far as the lock up goes, I’ll continue to document my findings and keep you and your readers informed. Thanks again for your input.

    Michael in Florida



  12. BB,
    Thanks for another great post. I’ve been stymied by the right to left shift you describe and now you’ve give me some info to help work on the problem.
    Much to ponder.
    Pestbgone



  13. Hey BB -
    Thanks for answering my question in such great detail! I didn’t know the adjustable mounts also adjust left and right along the x axis. The description of the spiral is also very interesting – sounds like a gyroscopic effect. If the spin from the rifling in the barrel does not stabilize the pellet as a powder weapon does – what is it’s function in the air rifle?

    I also did notice that some pellets do shoot left or right, and you say that’s normal, what is the mechanism in play there?

    Ozark


  14. Michael and BB,

    I have puzzled over the CO2 diagram even after reading http://www.warpig.com/paintball/technical/gasses/co2dynamics.shtml. After hearing Michael’s experience however, I have an idea.

    What I understand from reading is that all lines under the “hump” have a mixture of liquid and gas. Temperature lines that intersect that hump have a constant pressure in the straight, horizontal portion. Where the line leaves the hump on the right might be the “supercritical” portion, described in the WARPIG article as “the dangerous region.” They also say that “Here the CO2 expands and fills the whole tank and behaves much like a liquid.”

    Notice that 90 degree line never intersects the hump. In fact, at the 34% fill point, the 90 degree line shows about 1000PSI. When I converted my RWS 850 to PCP, BB figured I was starting to valve lock around 1050PSI because of my velocities. CO2 might valve lock before this since it is a thicker gas.

    Here is my theory. I’m guessing that a 90 degree day won’t usually raise the temperature of the CO2 bottle to 90 degrees unless the gun is left out for a while (I don’t know how long). When you are shooting the gun, it will keep the CO2 bottle cooled below ambient temperature and cause no valve lock. Perhaps in a humid environment you have more of the ambient temperature transerred to the CO2 bottle thereby warming it faster and causing valve lock? I don’t think this is probable because the behavior happens after a number of shots (which should cool the bottle). Perhaps some of the humid air is condensing and freezing in the transfer port or elsewhere inside and is causing blockage? After warming up, it then shoots for a few more shots.

    In any case it seems that if the CO2 bottle’s real temperature is around 90 degrees, you might be close to valve lock.

    Thanks,
    .22 multi-shot


  15. Ozark,

    Rifling in airgun is part of what stabilizes the pellet, as well as drag. And drag also stabilizes bullets from firearms, just not to the same extent as pellets.

    As for some pellets shooting left or right, I don’t know the answer. I assume part of it has to do with dwell time in the bore. Each bullet emerges from the muzzle at a different time in the vibration/recoil cycle.

    Why do firearm bullets exhibit the same behavior? My 10/22 groups left and right as well as up and down with different ammo at 50 yards. Like I said, I don’t know the full reason for it.

    B.B.


  16. BB:

    Regarding your answer to Ozark:”
    As for some pellets shooting left or right, I don’t know the answer. I assume part of it has to do with dwell time in the bore. Each bullet emerges from the muzzle at a different time in the vibration/recoil cycle.”

    Do you think that lubing pellets changes the dwell time in some pellets giving better groups? When I have lubed 0.22 CP’s with KryTech in my Diana they perform much better.

    KTK


  17. KTK,

    The little velocity testing I’ve done with lubed vs. unlubed pellets suggests that lubing slows them down. Of course once you fire a lubed pellet in a gun the bore is considered lubed, so you can go back without cleaning. That’s why I’ve done so little testing.

    Anything that changes the velocity of a pellet should change the point of impact.

    B.B.


  18. Hello BB, I posted this comment in an earlier post (the 2005 archive?), as i am not sure you respond to those “older posts” i wanted to post this in an updated post, so here goes, This is my first foray into “big bore airguns” I am purchasing a rifle that is capable of 470 foot pounds with a 205 grain “bullet”, I have a few questions, if i will be working with ranges up to 120 or so yards, what would be a good starting “zero point” so that I get the maximum amount of “leeway” such as the 20 yard inital zero point for a 850fps diablo pellet? The “bullet” I would like to shoot has a ballistic coefficient of .189 which of course has less drag than your standard diablo pellet, the rifle in question can shoot the 230 grain “bullet” at around 950 fps. I say approximate because the builder of the rifle at the present did not test a 230grain, but the gun will shoot a 205 grain conical at 1020fps, so by my calculation it should shoot the 230 grain at around 950-980fps for a final energy of around 500 foot pounds or so. My second question is, if the rifle can shoot a 205 grain conical at 1020 fps would it shoot a 230 grain at about 950-980 fps? Any help would be appreciated thanks, BTW this blog has a lot of really good info. This blog and of course pyramidair is responsible for me getting back into airguns again thanks.


  19. chev,

    You don’t have the same two zero points as a smallbore because you are zereoing so far from the muzzle that the sight/bore parallax is resolved. Since the ballistics don’t exist as tables, all anyone can do is guess. That’s what the ballistic software does, anyway.

    I would try a zero at 100 yards. That should get you within a few inches at 120 and keep you pretty much on at 90.

    However, a better way is to sight closer and learn the trajectory of your rifle. Unless you are shooting on a known distance range, most targets won’t be at 120 yards. You’ve just established that arbitrarily as a good “far point” for your gun. I would suggest zeroing at 60 yards, because most of your shots won’t be farther than that unless you force them. I assume you are intending to hunt game, and a 90-yard shot is a long-range shot that should only be taken after a shooter has experienced hundreds of shots with his rifle.

    B.B.



  20. KTK,

    Lubing pellets doesn’t affect accuracy directly, but it does affect it indirectly, by keeping the bore cleaner longer. Of course you only have to lube Crosman Premiers (at any velocity) and all pure lead pellets at over 950 f.p.s.

    What affects accuracy more than anything is sorting pellets by weight. Sorted into groups to the nearest 1/10-grain, they will group much better than unsortedf pellets of the same brand.

    B.B.


  21. BB,
    On an “Off-Axis” problem, how do you tell when your SCOPE is fully centered so you can make adjustments with the scope mount? Do you just get it close, or is there any way to check it on a scope that doesn’t have a “neutral” setting? JP



  22. I have noticed this same spiraling effect with a .22 rimfire. I have a bull barrel 10-22 and a target scope and can see the projectile in flight and it starts an ever widening spiral after 70 or 80 yards. I blame this on loss of velocity and a natural following of the twist on the bullet.


  23. Dr. Rahl,

    What you are seeing is called precession, I believe. It results from instability in a spin-stabilized object.

    I have never seen spiraling with a .22 rimfire bullet, but I don’t see why it isn’t possible, especially given the longer ranges you shoot at.

    Maybe I’ll have to do some 100-yard shooting when the sun is behind me.

    B.B.


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