AusVarmint

Analysis of Slow Motion all-of-flight videos; what can they tell us?
Pellet flight and spiral stats ... some observations from SM video

Ted from Madison introduced us to the camera and bracket hardware that fits to our scopes to allow through-the- scope video. Thanks again Ted. ... However most if not all such videos being shown on the internet only show the pellet in the last stages of flight.

I have been concentrating on developing the system to allow for all-of-range-flight depiction and analysis. The results have enabled me to get insights into some, or all, of the following analyses to the degree allowed by the quality of video made with a relatively cheap camera system in daylight.

A. Firstly some of the reference points of interest for me to note in the pellet's flight are:

1. The near zero where the pellet first crosses the line of sight
2. The far zero where the pellet again crosses the line of sight - (the shooter's zero)
3. The trajectory apogee where the pellet reaches the apparent highest point in its trajectory relative to the line of sight.

B. With reference to Dave's Chairgun CGBallistica each of these can be defined in terms of:

1. The time of flight.
2. The location of the pellet in flight between muzzle and target at any instant.

C. These in turn can be related to:

1. The extent of "clean" stable flight down range; and the point where it may become unstable.
2. The onset of spiral flight, if any, and its relationship to A1,2,3 and B1,2.
3. The number of spiraled flight rotations, if present; their direction, frequency per second and per unit distance; and relationship to initial pellet spin rpm .
4. Spiral flight's onset in relation to positive and negative acceleration due to gravity (climbing and descending).
5. Precession /oscillations in the absence of spiral flight (if light and camera settings allow that degree of definition).

Here is one example of flight, shown from the rifle to 71 yards, and some analysis to follow; MV 880 fps with a pellet that became unstable down range:
All videos probably best viewed full screen (click bottom right corner of video, replay if it starts early):

http://smg.photobucket.com/albums/v392/ ... om35yd.mp4

In our example we see the pellet first crosses the reticule cross hair at approx 10 yd and reaches the apogee at about 41 yards (distances derived in conjunction with Chairgun); it can then be seen in the video that spiral flight begins at around 35 yards/105 ft. Many pellets that apparently begin well and with stable flight, seem to lose it at about that range/ time of flight and we describe them as "25 or 30 yard pellets".

Approximately 5 left hand spirals from the right hand twist barrel in the last approx 36 yards of flight, can be counted in this example. Five spirals in the last 36 yards of flight equals an average of one spiral every 7+ yards ie., one every 21+ feet of forward travel.

Because the pellet takes about 0.146 seconds (Chairgun CGB) to travel the 36 yards from 35 yards to 71 yards, the rate of spiral would be approx 0.146/5 = one spiral per 0.0292 seconds. Say a rate of 34+ spirals per second, 2,040/min.

As this shot's pellet spin rate (from rifling) is approx 586 rev / sec, 34,800 rpm, (disregarding slight radial velocity decay) then the ratio of spin to spiral is approx 586:34 = 17+ pellet revolutions per each spiral.

Interesting to note that the direction of the spirals is always in oppositional rotation to the pellet's spin direction. Right twist equates with left spiral and vice versa. I have both L and R twist barrels and this always results. No, it has nothing to do with water and sink holes.

My slow motion videos do show that some pellets start spiraling right out of the barrel. The bullet-like old Sheridan .20 cal Bantam is an example which I have videoed numerous times.

http://smg.photobucket.com/albums/v392/ ... 25yd-1.mp4

Contrast that with this from JSB .20 cal Exacts shot from the same Sheridan rifle:

http://smg.photobucket.com/albums/v392/ ... 25yd-1.mp4

Further, some videos capture oscillations / precession without spiral (seemingly perhaps related to pellet centre of gravity where the forces acting upon the head are somehow counteracted by the drag at the rear), example: - Best viewed full screen:

http://smg.photobucket.com/albums/v392/ ... 1yards.mp4

The practical application of these analyses comes through correlating the above pellet flight quality/characteristics from such full flight slow motion video with:

* group size
* pellet muzzle velocity
* velocity decay rate - drag coefficient and ballistic coefficient BC (as tested for the particular rifle and pellet).
* points on the trajectory curve as they relate to positive and negative gravity to pellet trajectory vectors
* pellet head size and shape (pointed/round/hollow)
* wind direction (head/tail/lateral)
* air density/temperature/altitude.
* and interactions between the above.

To leave the reader with a positive visual impression of clean pellet flight barrel to target, and therefore an optimistic outlook, here is a barrel to target 71 yards pellet flight from a rare "pointed" pellet that does fly true - from a .22 BSA barrel even in a touch of breeze: 3 JSB .22 cal Stratons making a 1/4" / 6.3 mm group at 71 yards:

http://smg.photobucket.com/albums/v392/ ... at71yd.mp4

And three .25 cal JSB Kings to 51 yards shot from a Swedish FX Smooth Twist barrel at a particular velocity, in a light breeze, making a group of about 0.10" / 2.5 mm and showing great stability:

http://smg.photobucket.com/albums/v392/ ... 2012-1.mp4

Hope there is something of interest for everyone here who read this far.
Kind regards, Harry.

I kept up with most of this but the proof is in the videos. Great article

You are a ledgend Harry keep up the good work.
Ol 55

wow that was amazing!

Very cool Harry, somehow you made inaccurate pellets interesting Also show's what JSB's and other good pellets are worth.

Ryan

Nice work. Very interesting.

While this is specifically related to airguns, I think the theory is applicable to all firearms. Sure, there's some differences but it's still a firearm hurling a projectile...

The Raven wrote:While this is specifically related to airguns, I think the theory is applicable to all firearms. Sure, there's some differences but it's still a firearm hurling a projectile...

Yes and yes. I'll test interest on the PB forum ........... But my camera set-up is not up to the task of handling PB recoil. But I'm working on it.
Kind regards, Harry.

Thanks Harry,

Always a good read!

I will look forward to PB testing if it happens.....

regards, curan

Really nice stuff Harry.

Have you thought of shooting your camera recordings at night to get a full flight video?

I only mention it as I have done a lot of night hunting of pests, and under a gun mounted spotlight I can trace the pellet all the way to the target usually. That has helped me to manage/adapt to changing conditions such as nightime air density and cooler temperatures, and on occasions higher altitudes than where I zero'd during the day.

Illumination from behind using a spotlight should show up quite nicely in slomo video capture.

Just a thought, if you are seeking to get more flight time for analysis....

Also I was interested in your evidence of spirals being opposite to rifling direction. Do you think this is evidence of gyroscopic forces at play, is it consistent with the way a bicycle wheel gyrates when spun and held in your hand by only one end of the short axle? I found it tends to be stable until you go to move forward a step, then it starts to want to gyrate.

.177 cal is less afflicted by the magnus moment effect, probably (in my opinion) due to its lesser pellet mass. Is a .25 cal pellet afflicted more than a .22 cal pellet? You have both calibres, what say you?
cheers
GS

greyskull wrote:Really nice stuff Harry.

Have you thought of shooting your camera recordings at night to get a full flight video?

I only mention it as I have done a lot of night hunting of pests, and under a gun mounted spotlight I can trace the pellet all the way to the target usually. That has helped me to manage/adapt to changing conditions such as nightime air density and cooler temperatures, and on occasions higher altitudes than where I zero'd during the day.

Illumination from behind using a spotlight should show up quite nicely in slomo video capture.

Just a thought, if you are seeking to get more flight time for analysis....

Also I was interested in your evidence of spirals being opposite to rifling direction. Do you think this is evidence of gyroscopic forces at play, is it consistent with the way a bicycle wheel gyrates when spun and held in your hand by only one end of the short axle? I found it tends to be stable until you go to move forward a step, then it starts to want to gyrate.

.177 cal is less afflicted by the magnus moment effect, probably (in my opinion) due to its lesser pellet mass. Is a .25 cal pellet afflicted more than a .22 cal pellet? You have both calibres, what say you?
cheers
GS

Yes, I've done some at night. I may return to it but presently prefer to use morning sunlight from behind. As you have seen, the strategies I have worked out do a good job of tracking from well within 10 yards and, though I haven't shown much of my longer range video, I can get fair quality to well beyond 100 yards. Spotlight use will not change the range of flight time I am getting.

Yes, the gyroscopic phenomena you imply are the basic reason IMO. I could conjecture as to the relevant forces at play but may do better when I have gathered more data.

The last question is difficult to answer from my data. My .25 barrels are no doubt as good as they come as are my .22s. I cannot really say that the .22 pellets have any less tendency to spiral. I am only just now able to work with a quality higher power in .177 but already have evidence of one highly regarded .177 pellet that seems to lose it well before 50 yards.

Mind you the amplitude of some of these spirals is quite small.
The one shown in that first video is possibly no bigger than two pellet diameters. You may have noticed that it SEEMS to diminish with range. The spiral amplitude is seen as superimposed upon the target (disc) 71 yards away as the background. Consequently the first spiral, when projected to the target, covers more than the later ones. The last spiral over the last 20 odd feet of flight really indicates to me an amplitude of about one to two pellet widths. (The square hole in the disc is from memory about 1.25 inches - I'd have to check that ).

So - some very small groups can still be shot at longer ranges with some pellets that spiral. When they all have the tendency, I think we see round hollow groups. If only say one in five spirals we may just see a flier outside the cluster of four.

I have enough questions, for which to seek answers, to keep me interested for some time yet. As you would realize, each requires fairly elaborate and time consuming preparations. And then when you get a reasoned analysis there is always someone who comes up and writes that he knew all that "in the seventies" . I wish these types would give us the benefit of their great understandings now and save us some time.
You know the types.
Thanks again for your considered response GS .......... Best regards, Harry.

Its always a pleasure, and illuminating to read your write ups Harry. Even if I thought I knew something before reading your reports I would always consider evidence over anectodes.

I sometimes miss shooting high power, since committing to 12 ft/lbs FT competition. Trouble being after the considerable effort and finnicky attention to detail required to get everything set up spot on, making a change doesn't seem worth it just for kicks. I mean there's more at stake in FT competition! ROFL

That is what I have my gas ram break barrels for I suppose. Just out of interest, I once shot a 4" group at 100 yards with a .22 springer. Just for kicks. I know I can get minute of bunny head at 70y+ offhand with the gasram .22 hunter I built, but have never managed to bench that at 100 yards to date. Just too much to do at work.

Much of what I originally considered to be standard "dogma" is now turned on its head thanks to you and your experiments over the better part of 6 years I have been reading your posts, so thankyou Harry, for methodically approaching things in your unique way.

Am still struggling to develop the DA compensation system. Work always sems to get in the way.

GS

Yrrah wrote:.........So - some very small groups can still be shot at longer ranges with some pellets that spiral. When they all have the tendency, I think we see round hollow groups. If only say one in five spirals we may just see a flier outside the cluster of four..........

Hi Harry,

From your comment above, I figure you have experienced both scenarios often enough to get the feeling mentioned above, but have you started narrowing the cause? Your thought makes perfect sense to me, I just wonder if you have experienced enough of it to draw some rough conclusions against certain pellets, barrel fit, etc (I'm assuming you are sizing, weighing, etc).

My other question (and GS may also be knowledgable in this area) is relating to "air strippers". Have you ever used one, and what do you think of the potential? They have me wondering to the possibilities, but I cannot recall you using one. Therefore I am doubting their worth.

Thanks for your continued posts. I always find your research fascinating. You've got me remembering your posts on indexing nicked pellet skirts now, and wondering about applications in this phase of your research. Keep it up Harry, and you may well get to be the first person I have heard of to legitimately wear out a barrel on an air rifle using lead pellets

regards, curan.

[quote="curan]
My other question (and GS may also be knowledgable in this area) is relating to "air strippers". Have you ever used one, and what do you think of the potential? They have me wondering to the possibilities, but I cannot recall you using one. Therefore I am doubting their worth.

[quote]

Can only speak to my own experience. I have used one when reviewing the new Dayststate Panther .177. It was accurate, and from what I gather from other close sources, an air stripper will be as accurate as a well made and fitted silencer, but badly adjusted can be worse than a naked crown. Most air strippers have an adjustable cone so you can get the gap to the crown correct for your pellet length. Its beyond me why anyone would use one when a good old fashioned muzzle brake that deflects 90 degrees will do more for taming the muzzle flip. Turbulent air doesn't cause our accuracy problems, as I believe Harry has shown it has more to do with pellet dynamic stability than " dirty air" out of the barrel.

I understand that silencers are not legal over where you are, so will not push that line of thought, just to say I have custom made mine to produce very accurate results. I would not voluntarily swap to an air stripper design over my current set up, although I do sometimes swap out for a 90 degree brake I have. IMHO strippers are a gimmick, and thats just my lone opinion too.

regardsGS

greyskull wrote:Can only speak to my own experience. I have used one when reviewing the new Dayststate Panther .177. It was accurate, and from what I gather from other close sources, an air stripper will be as accurate as a well made and fitted silencer, but badly adjusted can be worse than a naked crown. Most air strippers have an adjustable cone so you can get the gap to the crown correct for your pellet length. Its beyond me why anyone would use one when a good old fashioned muzzle brake that deflects 90 degrees will do more for taming the muzzle flip. Turbulent air doesn't cause our accuracy problems, as I believe Harry has shown it has more to do with pellet dynamic stability than " dirty air" out of the barrel.

I understand that silencers are not legal over where you are, so will not push that line of thought, just to say I have custom made mine to produce very accurate results. I would not voluntarily swap to an air stripper design over my current set up, although I do sometimes swap out for a 90 degree brake I have. IMHO strippers are a gimmick, and thats just my lone opinion too.

regardsGS

I heard that discharge was louder with a stripper (so i figured there was no silencer issue to deal with) but was interested in the turbulent air theory rather than muzzle flip. Thinking about what you've said, and particularly about Harry's pellet dynamic stability results, I reckon I'll agree with your thought that it isn't turbulent air.

Thanks for the reply. It's saved me taking a path that I probably would have wasted a bit of time on.

regards, curan

Oh, and when and where are the FT world titles being held in NZ now? I'm having a good hard think about coming for a look (work, wealth and wife permitting).