BC obviously matters, but how much difference do relatively minor (e.g., +/-20%) variations really make?
This plot of POI vs range for BC varying from 0.020 to 0.030 suggests some perspective.
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Match pellets (wad cutters), have what would be reguarded as a poor BC. But produce very good accuracy.
Agreed. But did you notice, James, how little difference BC variations make...
...at ranges less than 20yds or so, anyway? Here's a blow-up of that region...
Thanks for posing the Plot, interesting to see the comparisons visually. My experience with the guns I shoot 100 yds with is: if I use different weight pellets (different BC) and keep MV fairly equal and zero at 50 yds I always end up using the same holdover mark on my scope at 100 yds.
The actual value of BC is of little importance to me but I do check BC as a diagnostic tool. I know about what BC a pellet should have when shot from a known good barrel at X velocity so if a test barrel shows a significant drop in BC when shot at the same velocity it is a good indication that the pellet is not being stabilized well and I can start looking for causes, crown, choke, bore, etc. Sometimes it can be improved, sometime I get another "mater stake". Measuring th BC can help narrow down the search of accuracy problems.
Hi Steve, Yep the short ranges don't show much difference. I had always assumed, that higher BC's would give better accuracy at longer ranges, all else being equal? PS. I'm in hammer bounce hell, with my bullpup build.
Would you be interested in trying an FFH?
Accuracy isn't really dependent on BC directly. Better BC pellets retain velocity better and because of that, show less wind drift. That's a help for the shooter's guesses at hold over and windage. Hold over won't really change (gravity being a constant and taget's are moving around on you changing the range), but it's a bit of help in wind-guessing.
Do think it's a help at longer ranges (like +80 yards)...the stability factor hasn't changed as drastically as a low BC pellets has (will add on a bit later).
But at moderate ranges, I'm not one to put those things as mechanical accuracy....more like help for the biologic part.
Accuracy isn't really dependent on BC directly.
Well, one scenario where BC does directly affect accuracy is when BC changes between zeroing and shooting. For example, suppose a gun is zeroed on a cool 50F day with a 0.025 BC pellet (typical of a CPH), but then shot on a hot 100F day. Because air density decreases about 1% for every 5F increase in temperature, density would be (100 - 50)/5 = 10% lower, making BC (which is inversely proportional to density) 10% higher.
So that 0.025BC would rise to 0.0275.
The graph predicts that, if MV were 900fps, the 100yd POI would therefore rise by about 1" from temperature effects alone.
I use BC for a relative comparison in selecting ammo. That is, I don't worry about BC when it comes to shooting what my air gun likes best if there is a very clear winner in regard to accuracy. But, if I have two (or more) pellets that shoot equally well, at say 20 yards, then I look at the numbers and shoot the one with the best BC to retain as much velocity and accuracy as possible at further distances.
I should also mention that I usually don't have to do much numerical comparing. I know from what I've read before that round nose pellets are almost always going to have a better BC than a wadcutter or a hollow point pellet. So, I always shoot round nose pellets if I find one that seems to shoot accurately. Now, with that said, if two or more round nose pellets shoot about the same as far as accuracy, then, again, I look at the BC's and choose the one with the best numerical value.
B.C. comes at a premium for the airgunner;we pay dearly for a small difference.You have shown what the effects of a small difference might be.It's not so much really.The long distance shot,and short distance high precision shots, are times when we should be most concerned.You have shown us, in previous YELLOW FORUM posts,that a 3% velocity change(or more) in a 100 yard shot(at abt.900 fps) can cause so much drop that it would ruin an otherwise round shot pattern by elongating it vertically.I have found that the CPH pellets you mentioned can vary in weight over a range of 1.3 grain.That weight variance alone can create a velocity variation of about 3%.And it is almost not possible to have that much weight change, without changing other factors relevant to the B.C.We are imparting too many significant digits in our B.C. figures for the errors we are allowing,and the too small sample size used in the testing.It just hasn't been that precise anyway.
The Piledriver design ,and a couple of others,are so far ahead, in terms of B.C.I don't understand why the airgun community in so unconcerned with them.So many people interested in accuracy and long distance shooting have spent a lot of money and time on guns that can shoot a low B.C.pellet well.Wouldn't we be well advised to find the barrel that would properly fit the Piledriver (both in size and in twist rate)and reap a better situation?
EricinMaine-
I agree that a higher BC is most useful in masking any errors the shooter makes in regards to windage calls. There are areas with larger variability that can be addressed. Also if you're serious about shooting at longer ranges, typically you'd verify your zero to alleviate any variation in your POI/POA relationship caused by environmental conditions.
B.C. comes at a premium for the airgunner;we pay dearly for a small difference.You have shown what the effects of a small difference might be.It's not so much really.The long distance shot,and short distance high precision shots, are times when we should be most concerned.You have shown us, in previous YELLOW FORUM posts,that a 3% velocity change(or more) in a 100 yard shot(at abt.900 fps) can cause so much drop that it would ruin an otherwise round shot pattern by elongating it vertically.I have found that the CPH pellets you mentioned can vary in weight over a range of 1.3 grain.That weight variance alone can create a velocity variation of about 3%.And it is almost not possible to have that much weight change, without changing other factors relevant to the B.C.We are imparting too many significant digits in our B.C. figures for the errors we are allowing,and the too small sample size used in the testing.It just hasn't been that precise anyway.
The Piledriver design ,and a couple of others,are so far ahead, in terms of B.C.I don't understand why the airgun community in so unconcerned with them.So many people interested in accuracy and long distance shooting have spent a lot of money and time on guns that can shoot a low B.C.pellet well.Wouldn't we be well advised to find the barrel that would properly fit the Piledriver (both in size and in twist rate)and reap a better situation?
EricinMaine-
There are a number of points here. First off the weight variation in pellets will produce a muzzle velocity change yes, but it will also produce a BC change which will somewhat offset the muzzle velocity change. The compensation in the total on target effect is greater for PCPs than it is for springers due to the basic propulsion system.
When testing bullet drag using tracking radar I always used to insist on 10 round serials fired on three different occasions to obtain an overall Mach number drag coefficient relationship. BC testing should use a similar number of rounds and only be conducted under known atmospheric conditions. Many of the quoted variations in measured BC values are I believe down to unknown atmospheric conditions during testing and not different barrels etc.
As for piledrivers, much of the BC seems to be due to the very high weight as opposed to a low drag coefficient. The derived drag coefficient from some of the measured BC values is not very impressive. To my eyes the boat tail design used on these pellets has a number of undesirable details which adversely affect the performance. I have also estimated the moments of inertia and the C of G position, none of which gave very promising results in terms of basic ballistic properties.
The piledriver is a commendable attempt at something different but I believe there is a lot of room for improvement.
Miles_M,
Steve shows that some of the range of B.C. we concern ourselves with is of questionably small consequence.I think his point is well taken.His presentation is based on the potential of actual ,accurate B.C. figures.I think the range he has shown is of little consequence also because I think it represents a range of inaccuracy in our B.C. figures.You seem to agree,and think the error range is due to widely varied atmospheric conditions that were not read,F or adjusted to standard atmospherics.I can agree with you .I also think that too much variation is left to chance because steps were not taken, in much of the testing, to ensure that the tested projectiles were accurate representations of the pellet design.In addition to attention to the atmospheric conditions,the weight of the test pellet must be as design specs.The head size and skirt size must be correct and consistant.And a visual inspedtion should be made to weed out defects.Some tests reports show that this was not done,Some others just don't mention anything about it.There are many reasons for using the B.C.figures,I think we all need them to be as accurate and believable as possible.
Now in regards to the Piledrivers...You are casting dark shadows over my hopes and dreams for them. 🙂 One report showed a B.C. of 0.033 at 30 yd. and at 641 fps.,and another report showed 0.095 at 75 yd. and at a particular spin rate(one I calculated I could achieve with my mrod at 620 fps.).These were both for the .177 cal.version.Atmospherics?Isn't the whole point moot if they don't fit my barrel ,and come out wobbly ?I'm hoping for the best.
EricinMaine-
Miles_M,
Now in regards to the Piledrivers...You are casting dark shadows over my hopes and dreams for them. 🙂 One report showed a B.C. of 0.033 at 30 yd. and at 641 fps.,and another report showed 0.095 at 75 yd. and at a particular spin rate(one I calculated I could achieve with my mrod at 620 fps.).These were both for the .177 cal.version.Atmospherics?Isn't the whole point moot if they don't fit my barrel ,and come out wobbly ?I'm hoping for the best.
EricinMaine-
Eric
Pellet/barrel fit is crucial to group size but to be honest I do not think in practice it makes a lot of difference to BC. The drag increase due to wobble is a function of yaw squared so small yaws produce very small changes in drag. However, due to the poor ballistic yawing behaviour of pellets, the piledriver included, any small yaw will give a large increase in group size. Therefore we would reject a pellet for poor precision long before the change in BC shows. The numbers just don't add up to give you have a pellet with a yaw large enough to affect the BC without a gross effect on group size.
I would say give piledrivers a try, they may work in your guns, they may just happen to fit. Just don't try them in a smooth twist barrel with its very low twist rate.
Miles
Miles_M,I did not know that drag increase due to wobble was a function of yaw squared.I pictured (in my mind)that the wobbling pellet effectively presented a much larger,and flatter face.I gave that effect too much value.I guess I should have given more consideration to the fact that at any instant in flight ,the actual size and shape are not changed.
Thank You for a clearer view,EricinMaine-
I think airgun ballistics need to be my next month long aspergers level endeavor to dive into. I'm totally unaware of any ballistic studies or practices in the airgun world at this time, only cf. However I have to agree with Mike M on all accounts. The strange thing with airguns is that I think it's going to be hard to move away from the front weighted diablos so as to achieve better BC. I'm sure a higher BC pellet with a change to the diablo design could produce better BC but the manufactures should, IMO, have to achieve consistent accuracy first to pull people away. Up to this point I've always just looked at a pellet and asked myself "if this pellet is accurate at 10 yards is it going to be accurate at 30?" and unfortunately I have guns that perform great at 10 yards but open up like crazy past that. If I was comparing 2 pellets that performed equally on target then I'd concern myself with BC more but the margins for my airguns is so dramatic between what pellet is accurate and what isn't I have yet to bother. As for the PLDVR design, just looking at it I feel like the high BC comes from the weight, the boat tail does seem more like a marketing ploy but like Sigmond Frued in psychology, some times it takes a new approach to get better at something in the long run so I wont knock it.
I think airgun ballistics need to be my next month long aspergers level endeavor to dive into. I'm totally unaware of any ballistic studies or practices in the airgun world at this time, only cf.
Alan
Having just retired after nearly 40 years of working exclusively in external ballistics it was only natural that I should study pellet flight from the theoretical and practical viewpoint as well. I have presented some of what I have done in threads on the airgunforum.co.uk (under the name ballisticboy) where I have also tried to explain some basic pellet properties. Be cautious of anything on youtube as, quite frankly, the vast majority of it is rubbish produced by people who do not know what they are talking about and who do not know how to carry out tests to rigorous scientific standards. As a result there is an awful lot of myth and misinformation around posing as facts.
On the subject of moving away from the diabolo shape there is a thread on the same forum on experiments I carried out with Gerald Cardew 30 odd years ago on different pellet designs, which I produced and Gerald made and tested. I have recently refined those designs and plan to continue the work when I have the facilities.
Thank you very much for providing to the field then. I'll see if I can find some of your writing this weekend. I am curious as to what work has been done so I'll for sure be looking. I've wondered about casting pellets since it seems easier then CF. I like the separation between theory and practical, like in the above example where results are seen in a much larger picture with accuracy before BC is a factor. Recently my 8yo daughter asked me why one pellet is more accurate out of a given gun... Generally I answer her questions in a very thorough method but after I gave a few reasons I just got the sense that I wasn't entirely sure why it is lol So I told her I'd have to look into it more. Right now I'm interested in pellet fit in the barrel, how the air hits the back of the pellet and the pellets response (per different power source), rifling, and pellet design then in comparing BCs. If I encounter any youtube info I'll be sure to take it as opinion and not get married to any ideas.
I think airgun ballistics need to be my next month long aspergers level endeavor to dive into. I'm totally unaware of any ballistic studies or practices in the airgun world at this time, only cf.
Well, just for a starter, here's the math that produced the plots that began this thread.
Drop_From_Boreline(inches) = (193*(34500*bc*(EXP(x/(11500*bc))-1)/v)^2)
x = range in yards
v = MV in fps
Thank you very much for providing to the field then. I'll see if I can find some of your writing this weekend.
Alan
The new website is not very user friendly here is a link to a list of threads. Not all of them are technical. Hope it works.
https://www.airgunforum.co.uk/community/index.php?search/128680/
Steve, You might have ruined my morning... it's 6am right now. lol
Miles, Thanks for the link, heading there now.
Regarding a 3% drop at 100 yards elongating a spread vertically... My only evidence is carefully monitoring other peoples groups on youtube where they're shooting targets and through a chrony. I've paused videos and taken notes..... So having said that it seems to me that when there is a drop or rise in velocity* it effects the spin out the barrel which doesn't just change spread vertically. I've seen slower pellets hit high and to the right, and faster pellets down and to the left. This is of coarse at sub 40yd distances. I have no practical or theoretical past that. It's for this and some other reasons I've been slow to enter into the field of airgun ballistics, too complex. But I do see the need to be informed. Not going to lie, I have no idea what I'm looking at with the above equation, a little more involved then calculating G1 BC lol. I'll get it by noon...
Steve, You might have ruined my morning... it's 6am right now. lol
Miles, Thanks for the link, heading there now.
Ouch! That certainly wasn't my intention, Alan!
Drop = (193*(34500*bc*(EXP(x/(11500*bc))-1)/v)^2)
What terms do I need to define? BC as used here is based on a simple 2-point retained velocity measurment. For example...
bc = (yards between V measurements)/(8000*LOGn(Vuprange/Vdownrange))
Drop = (193*(34500*bc*(EXP(x/(11500*bc))-1)/v)^2)
What terms do I need to define? BC as used here is based on a simple 2-point retained velocity measurment. For example...
bc = (yards between V measurements)/(8000*LOGn(Vuprange/Vdownrange))
A note about calculating BC with this simple (simple by ballistic calculation standards) formula.
bc = (near-far_yards)/(8000*LOGn(Vnear/Vfar))
It's predicated on a constant Cd model (drag force proportional to V^2), and agrees closely with G1 calculators for velocities that are solidly subsonic: V < 1,000fps.
It predicts a velocity loss rate of: V/8000/BC per yard.
Stated more formally...
dV/dx = V/8000/BC, x in yards
Tags
Ballistics · Accuracy