Longer range shooting – thinking aloud and putting into practice.

Forums General Airgunning Longer range shooting – thinking aloud and putting into practice.

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    Yrrah
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    A few things to consider:
    The farther the range the better the drag coefficient (Cd) and ballistic coefficient (bc)  need to be in order to sustain velocity and so the amount of energy required at point of impact (poi).

    The stronger the winds  the better the Cd and bc should be to counter wind drift and head wind pressure.

    The slower the muzzle velocity below that at which best accuracy/precision is achieved for the rifle/pellet combination the greater is the wind influence; and the the more critical is the sight/holdover adjustment required for precision shooting.

    The more powerful the rifle/ pellet combination, the more critical it is for the shooter’s skills to hold together especially during triggering and follow – through.

    These factors, though not all inclusive, become increasingly significant when seeking accuracy and poa/poi coincidence at increasing range especially under adverse environmental influences.

    The best ballistic coefficients, for the best pellets in the smaller calibres, that I have measured from my rifles run something like this for “average” conditions at my farm but summer/ winter need tweeking.
    .177 – 0.028 for 13.43 gr JSB Monsters and .09 Prometheus Piledrivers.
    .22 – 0.034 for JSB 18.1 gr Exact Heavies; 0.048 for new Revised 25.4 gr Monster JSB;  0.08 for Prometheus 30 gr Piledrivers.
    .25 –  0.05+ for JSB 33.9 gr King Heavies Mk 1 ; 0.038 for normal 25.4 gr Kings JSB.

    All my rifles are super accurate for that for which they are designed. All are run at the optimal mv for the above pellets.
    So, with the above points of consideration in mind,  I have a sound base for deciding which calibre and which pellets may be best for the job at hand.

    This morning at our farm it was a little breezy and as it is rather open grazing land,  I took the .25 FX Crown  and a magazine of 33.9 gr JSB King Heavies Mk 1 skirt sized to 0.2515″ (to easily load the magazine) and lubricated with a wax. Shooting at 850 fps, which I have found to be in the middle of the best Cd and accuracy range using the LabRadar,  I took a stroll – walk a little, look a lot- and enjoyed the early morning crisp autumn air.  Three shots, one at 87 yd from a tree branch lean; a second at 48 yd also from a tree branch; and a third also head shot at 131 yd off the top of a fence strainer post was a very satisfactory morning. The last was waited out for a lull in the movement of the grass. No hurry, the bunny was warming itself in the early morning sun. I use an old Bushnell rangefinder and Chairgun calibrated for my 3,640 ft altitude and prevailing temperature.

    The Crown has now shot 219 rabbits off this property when we have visited since December. They are pretty much back under control now.

    Life is good. I hope some of the newer shooters may get something to think through from this.
    ​Best regards to all, Harry in OZ.

    • This topic was modified 7 months ago by Yrrah.
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    zx10wall
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    Thank you for sharing sir. 

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    AirSupply
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    Hi Harry. Always enjoy your posts. 219 is a great telly. How did you go with the King Heavy mk11 pellets. I didn’t have great results in my Crown. I have some mk1 Heavies that I haven’t yet tried. Sounds like they may be a better option?
    Michael

    • This reply was modified 7 months ago by AirSupply.
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    Yrrah
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    Michael,
    The Mk 11s  also shoot very well but I have been very sparing with them so far. 
    Both drift less in wind as readers will infer from the OP.
    This rifle also shoots old Baracudas very acceptably and loves the Predators which I get a kick out of using in the field.
    I hope all iswell down south.  Best regards, Harry.
     

    • This reply was modified 7 months ago by Yrrah.
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    mubhaur
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    Dear Harry,

    How do you compare the accuracy of crown vs smooth twist vs LW vs BSA barrel?

    Now as you have all these only you can give real input.

    I sti’ll believe that maximum aaccuracy potential is somewhere between 850 fps and 910 fps.

    What are the exclusive characteristics of  all these 4 kinds of barrels?

    I am tempted towards RAW now with polygonal barrel. 

    Regars,

    Umair Bhaur 

    • This reply was modified 7 months ago by mubhaur.
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    Diamondback
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    Brother Yrrah,

    I’ve been shooting some of my classics a bit more lately and have really enjoyed them. You’re wonderful posts helped motivate me to purchase my classics years ago.

    Lately I find myself wanting to try something like an FX Impact or Crown, or a RAW, or ??, and then you share this encouraging post. 

    You always hit the mark, and not just with projectiles.

    Great post as always,
    Mike

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    jking
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    I think it was Ted that was talking about BC one day and I “think” he stated that the BC changed enough going to a lower velocity that that is where he tuned to. I think it was when he was telling us how he won the EBR two years ago. Again, I think he said it bucked the wind better at the lower velocity, higher BC. It’s always stuck in the back of my mind, yeah way back there somewhere. I was wondering how a pellet fired from a velocity of 880fps could have a lower BC than that same pellet fired at 830fps. I guess it would be easy enough to check out with a few pellets and a chrony. It could probably be proved/disproved by just tinkering with Chairgun.
    Enlighten me guys…
    Jimmy

    • This reply was modified 7 months ago by jking.
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    Centercut
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    There is much good information on this at GTA on Bob and LLoyd’s Workshop… Search “ballistic coefficients changing with velocity”.  In a nutshell, and depending on pellet weight (since most pellets are diabolo shape and are about the same), the BC will be in a bell curve with velocity on the horizontal axis and BC on the vertical axis.  So for your “usual” 16 or 18 grain .22 pellet, BC will start at a value around .020 or so at 500 fps, and go up to close to .040 at around 800 to 850 fps, then start heading down again until it goes below .025 (and further less) at 950 fps. So depending on weight, size and shape, the BC will start low, travel up in a bell curve shape, then head down after a certain velocity.  Apparently Ted found by experimenting and shooting many thousands of rounds what velocity his “best BC” was located for his pellet and his gun/barrel combination. For the 18.1 grain JSB this appears to be near 850 to 860 fps with a LW airgun barrel. FYI, I have noticed that my DS Renegade seems to shoot most accurately right around that speed range, and have it set to 860 to 865 fps.  I’ve tried all speeds in 20 fps increments from 800 up to 900, and 860 seems to definitely work the best, for me anyway… As weight gets heavier, the BC curve shifts to the right, so the peak BC would/should be a higher velocity. I think that’s why people seem to be able to shoot the heavier pellets (.25 JSB 34 gr) at higher velocities, like the EDgun Matador with Huma Power tune at 900 to 920 fps, or the Kalibrgun Cricket with Charlie Freer power tune at the same velocities, yet still retain accuracy. Yes, this is significantly higher than the “Ted velocity”, but the guns just mentioned do not have a ST barrel, they have the LW barrel, which could explain the difference…


     

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    Yrrah
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    Jimmy,   This is long and I am in a completely different time zone to you so please forgive the extent of it.

    The original research on BC changes with velocity change for airgun pellets is in this  report that I wrote in February 2010 which included the data collected over 50, 100 and 200 yards – and which shows the incremental changes for velocity and range breakdown – can be googled and read in this report if you enter the following precisely for a search. I also reported it then on the Yellow Forum. Unfortunately the pictures and graphs are not available from the host now.
    This data and follow up data that I supplied to Dave Eades formed part of the base for his Chairgun GA input.
    Google:  ”  Promised 200 yards pellet Ballistic Coefficient experiment data and results. LONG 

    I hope you enjoy reading the report which details the research method, ambient conditions etc., and the raw BC data as well as that converted to standard conditions.

    More recently I have filled in the nearer range data muzzle to 70 yards using my LabRadar which I have had for nearing 3 years.

    But to briefly respond to your question:  There are a number of factors which influence the changes depending on the particular pellet and the velocity projected; and also the point down range in relation to the pellet’s trajectory profile. This could be a long response so initially could I just summarize points and maybe go from there if there is further interest? 

     Pellet stability out of the gun is I think understood by most to be a factor.  Good pellets will stabilize in the first 25 to 100 yards. Until statically and dynamically stable they present with greater frontal profile and so higher drag and therefore lower effective BC. The above research seems to highlight that phenomenon. As good pellets settle the drag coefficient improves. However some pellets are unable to stabilize completely and just suffer worsening drag and BC.
    Which brings us to spin rate per unit travel:  Though diabolo shaped pellets rely heavily on flare stabilization and drag,  particularly early in flight spin rate per unit travel appears to be important. Spin rate per unit travel is influenced by velocity.  From a given barrel with a given rifling twist rate, the spin rate per unit travel is higher if the velocity is high and vice versa.
    Now if we put that into the context of the current question, we might suspect that at very low velocities the spin rate per unit travel may not be sufficient for optimal stability to be achieved. Hence as we up the muzzle velocity initial stability is improved and therefore a better drag coefficient (Cd) and bc is to be found. To put that into real world context, a JSB King shot over my LabRadar shows improved bc from <500 fps from my Crown to approx 800 fps. It attains best bc at around 830 fps .  This it maintains to approx 900 fps after which higher muzzle velocities show deteriorating bc, not much, but after 940 or so fps mv some pellets begin to spiral.
    Why should this happen?
    Well it appears that the air flow over and around a pellet borders on the speed of sound when the pellet to gound speed is still at about Mach 0.7 to 0.9 even though the pellet speed relative to ground is still well below the speed of sound.  This is where a loss of stability becomes more and more apparent until at some critical value the pellet loses its ability to stabilize. Thus we have a window of mv values (for each pellet according to its geometry, round head, pointed, flat nose, long body, degree of skirt flare, light head etc) for best mv and initial stabilizing for each pellet.

    Now we have to put this into practical shooting sense.  We want the best compromise between acceptable down range velocities for the task and stable accurate flight.
    Ideally we would begin with an mv as high as possible within the best bc range so that the benefits of velocity and energy can be sustained as far down range as possible before instability may set in.  However, once we have established sound stable flight in the early stages then we have that flare stabilization factor up our sleeve to keep the pellet on course even when it may have dropped below the velocity at which it would not have stabilized right out of the muzzle.

    There is much else to be considered such as the tractability of the pellet which can keep its axis following the trajectory curve when it passes the apex of trajectory.  If it is overspun in relation to spin rate per unit of travel then the overstability at that point down range will not allow for optimal tractability.  Gravity will fight with the pellet’s desire to follow Newton Law and the resultant moment forces will cause the pellet to become unstable again and suffer deteriorating bc and Cd and in many cases result in spiral flight.  I have shown this to be so many times in sm video relating the instant when correlating the video to the instant velocity as shown with the LabRadar.

    I guess the upshot is that for each good pellet and rifle we have a best window of muzzle velocity for best bc/Cd  and therefore range limits for accuracy. We are fortunate that by design or by accident and evolution we have some few pellets that allow accuracy at long range if we abide by the dynamics involved. Personally, since first shooting and reporting  sub inch 100 yards groups 15 years ago, I have never found any .177, .22 or .25 pellets other than  certain JSBs that will consistently cut the mustard at and beyond 100 yards.

    That is enough for now Jimmy and other readers who may have hung with me thus far, but I hope you open that research post referenced above as it was a landmark in this saga. …  Very best regards, Harry.

     

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    PerkyVal
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    Is it possible that at higher fps the spin drift has more effect on accuracy, especially with guns having a faster twist rate? If so is that one reason we see better accuracy at lower speeds?

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    Yrrah
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    “PerkyVal”Is it possible that at higher fps the spin drift has more effect on accuracy, especially with guns having a faster twist rate? If so is that one reason we see better accuracy at lower speeds?

    
At 200 and 300 yards, with certain pellets, I see a tendency towards spin drift. But it is so difficult to get the right conditions to really come to a quantifiable conclusion.
    The stability at those ranges is very biased towards flare stability, but one would think there could still be spin drift even at the relatively low rpm of pellets compared to cf rifles. Rimfire rifles however do show spin drift with sub sonic target bullets at 200 and 300 yd but their rate of radial velocity decay is much less than our best pellets.

    I have measured and reported spin rate per unit distance traveled out to 200 yards and the figure showed approx 1:12.8  inches from an initial 1:18.9 inches at 2 feet. The rate increased progressively from 2 ft to 75 yards to 100 to 150 to 200 yards.  I used dye marked pellets shot through two screens of known separation and over a chronograph to measure rotation as best I could.
    The rpm of course is less at that range but its decay is at a lesser rate than is its linear velocity; which accounts for the climb in rate per inches. … regards, Harry.

    If any are interested you can read the spin rate research by googling this topic:
    Pellet spin rate increase with range to 200 yards study and data … long with graphics.“.      Edit by Harry.

    • This reply was modified 7 months ago by Yrrah.
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    jking
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    Wow Harry, thanks so much for spending the time to reply and also the time you put in on your previous test back in 2010. Funny, then the 25.4’s were the “new kid on the block” and now its the Heavies. I’m speculating that it’ll be a line of slugs next, maybe with a JSB brand on them. I think I have a better understanding now of BC in relation to velocity and distance. I was surprised that the BC actually increased at even further distances. I’m sure I’ll re-read this a few more times so that bulb will shine a few more times.
    Thanks also Centercut for the chart and text.
    Jimmy

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    Centercut
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    Jimmy, you’re welcome but compared to Harry my explanation was more 5th grade than graduate level. I read both the reports Harry referenced, since I like that sort of technical stuff. Just the concept of BC and how it varies with pellet shape, spin, speed, and distance is fascinating. I think my next Christmas present to myself will be a LabRadar unit…  
    ;)
    Mike

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