Ballistics Coefficients Discussion

[Scotchmo]

In fact I do this on my data.... If I am experiencing a 1% ES and I get a shot that is 1% from the average velocity and therefore well outside the dataset, I discard it and shoot another one, which is almost always within the dataset.... If that replacement shot is "just" outside the next slowest or fastest, I keep it.... I know, it's a subjective choice, but it's just the way I do it.... I agree it is often a damaged or renegade weight pellet....

Bob

That approach would have made my last data set look very reasonable. I should have just shot more but I thought 10 was enough. For now, I'm dropping the theory of the side wind significantly impacting the BC.

Down range velocity:
661.4
659.3
674.8
650.3
667.3
666.8
626.9
625.6
627.1
674.7

Those don't look right. I should have thrown them out.
For now, I'm going to attribute the three outliers to some unknown anomaly or just bad luck.

[rsterne]

Kamakaze Mosquitoes....

Bob

[Scott Endler]

I have only measured the BC of the .177 Crosman Premiere Light brown box pellets twice and came up with .0190 over 33 yards from my Challenger at 790 fps and .0230 from my Talon SS over 12 yards and 840 fps.
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A 24 inch .177 barrel for the Talon and 2 chronies (maybe 3) would easily get a full set of data  from 1,400 fps down on one tank.
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Playing with Chairgun using the GA model and .0230 for that pellet and a 10 mph side wind shows  increasing wins drifts from 5.00 inches at 900 fps to 5.34 at 1,000 fps to 5.90 at 1,100 to 6.56 at 1,200 fps. Just because of the short period of time of the flight where the BC is so much worse in the trans sonic region. Even tough the total time of flight is so much less. .163 seconds at 1,200fps vs .196 at 900.
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Don't bother shooting airgun pellets above 900 fps.

[rsterne]

You've got it exactly right.... and the new model, when we develop it, will show that to be the case even more, because the GA Model doesn't have a large enough difference in drag between subsonic and supersonic....

Bob

[Pellgunfun]

I've got a question about BCs.

How can two pellets that have the same weight and shape have different BCs???

Two pellets that I had been playing around with.

The CPHP @ 4.3 gr.
The JSB Express Exact @ 14.35 gr
Both in .22 cal.

I can't imagine the 0.05 gr would make any difference at all.  These two pellets look the same have the same shape, and are the same weight.  They also produce the same muzzle velocity of 891 in one of my rifles.  According to Chairgun they have very different BCs.  How can this be?

[rsterne]

The BC's may have been measured at different velocities.... Additionally, it only takes a tiny difference in shape to change the BC.... Certainly the HP on the Crosman wouldn't help.... When I measured the Express at 923 fps and Premier (solid) at 922 fps, I got 0.023 for the JSB and 0.020 for the Crosman.... not a huge difference, but a difference, and it wasn't the velocity.... The H&N FTT, which is a similar shape, had a BC of only 0.015 at 912 fps.... It is also quite possible that the shape after firing, when the skirt has expanded, may be quite different between the three pellets.... 



Bob

[Pellgunfun]

The more I look at the more I think it has to do with the wasp waist.  Pellets of the same weight, and head size will have different BCs based on how skinny the waist section is.

I've noticed that pellets with a fat waist have a better BC than pellets with a much thinner wasp waist.  Seems that pellets with almost no wasp waist have better performance than pellets with a super tiny wasp waist.  Maybe that is why.  Now if someone would produce a diablo wasp waist pellet that almost no wasp waist at all.  That might be the best pellet for sure.  Has anyone tried to produce pellets with a very low about of wasp waist?

[QVTom]

There is the JSB Monster and the th H&N Rabbit Mag.  Both have a reputation for poor accuracy.  BC is great but hitting your target is better

[rsterne]

Actually, the Monsters have a good BC only because of their weight.... Factor out the weight, and the FF isn't as good as their lighter cousins.... No idea why....

Bob

[MicErs]

I don't know why a boolit shaped projectile much like the old BP Minnie' ball would not be a good fit in modern rifled AGs.  The velocities are about the same really.  The wasp waisted pellets were a concession to smooth bore rifles of the 1800's and early 1900's.  Why we kept that shape has never really made a lot of sense to me.  If you can flip a pellet 'round backwards and it manages to stay in that orientation all the way to a fifteen or twenty yard target because of the rifling, the the waist serves no function.  The hollow base, sure...

It would seem to me that it would be easier for vendors to form a hollow base spitzer pellet than a wasp waisted pellet anyway.....

[rsterne]

Yep, they work just fine, in the more powerful PCPs.... Mike (Carson Rat Sniper) is selling hollow based spitzers in .25 cal in a few weights, from 31 gr. up to about 45 gr.... Most big-bore shooters just use cast bullets, either RN or FN designs....

Bob

[Scott Endler]

I've got a question about BCs.

How can two pellets that have the same weight and shape have different BCs???

Two pellets that I had been playing around with.

The CPHP @ 14.3 gr.
The JSB Express Exact @ 14.35 gr

One is a hollow point and one is a dome. They are not even close to the same shape. What's really weird is when the same pellet has a different BC out of two different guns. And that BC drops off with increasing range. Due to an increasing nose up attack and the onset of spiraling?

[rsterne]

Actually, depending on the muzzle velocity, the BC often increases with range, because the velocity slows into a region where the Drag Coefficient is lower.... This is particularly true if you start at 1000 fps or beyond.... However, that is only because the Drag models we typically use (G1 or GA) do not reflect the huge increase in drag at transonic velocities.... If the model matched the drag, then the BC would be a constant, even though the large increase in drag (and loss of velocity) would still be taking place.... I dealt with that a couple of pages ago....

Pellets with a soft, thin skirt can deform quite differently at different pressures, as the skirt obturates to fill the rifling.... This change in shape can easily affect the BC when shot from different guns.... The increasing nose-up angle of attack is very small, even at 100 yards, that was touched on in an earlier post in this thread.... In fact, if the rifling twist is not too fast for the pellet, it doesn't occur, and the nose of the pellet follows the trajectory.... Spiraling can increase or decrease, depending on if the pellet is dynamically stable or not.... If it is, the spiraling decreases with range, if unstable, it increases.... There is a thread in the PCP gate dealing with those cases at this point in time....

Bob

[Pellgunfun]

Why wouldn't this pellet design be one of the best for rifled barrel pellet rifles.  Especially in the .177, and .22 cal lines.  Is it true that this one was only available in .20 cal?  Was it ever available in .22 cal?

It has a very slight diablo style waist, that basically allows two rings of contact with the rifling.  Seems sorta silly that the air pellet industry has held on to tiny waist high drag pellet design.  Especially when just about every pellet rifle these days has a rifled barrel that can spin and stabilize the pellet instead of depending on a 'birdie' style wasp waist design to stabilize pellets fired from smooth bore barrels.

[rsterne]

You would have to answer your own question by testing it at various velocities to find out what the actual BC is.... My guess is that the relatively pointed nose, compared to a true hemi-spherical nose like the JSB Exacts, will hurt the BC more than having a larger waist might help it.... As I said above, I haven't found a larger waist to improve things much, if any....

For the most part, the BC is tied to the SD, which is the weight in lbs. divided by the caliber squared.... In fact, the BC = SD / FF , and the Form Factor doesn't vary as much as you might think, being 1.0 for the 3 caliber long spritzer G1 Model (1st page), and 1.55 for a roundball, with most good pellets lying in between.... Wadcutters are worse, of course, and so are most pointed pellets, and domes that are not hemispheres, because of the sharp corner at the back of the head.... Weight is still the most important contributor to BC....

Part of the reason to hold onto the Diabolo design may be the desire to limit maximum range, and the terminal velocity and energy at that range, for downrange safety on high angle shots.... If you want to really allow an airgun to stretch it's legs, forget about pellets, and use a good bullet design, in a barrel chambered for it, with enough power to push it.... .22LR performance is available in modern PCPs....

Bob

[Pellgunfun]

I think that a good 'bullet' design is in order here.  I'd love to see a line of air rifle 'bullets' in .22 cal.  Could be just what the sport needs.  As you said, I'd love to have these air rifles be able to really stretch their legs.  As I said in another thread I really like your 'boat tail' design.  I think something that could work well.  Is it possible to produce that bullet in a lighter weight?  Has anyone considered some sort of hollow core in order to reduce the weight, but still get the right profile?

[rsterne]

Boattails are not great in short, and therefore light bullets.... The bearing area needed to keep the bullet straight in the bore simply gets too short.... Since the weight is the dominant factor in the BC and not the shape, there is no more point in making a hollow bullet than one made from zinc, like the lightweight pellet designs around today.... It might be possible to make, say, an 18 gr. bullet with a fractionally higher BC than, for example a JSB Exact Heavy.... but I don't think you will find anyone willing to put in the effort to make something like that because of the cost.... Any type of hand casting or swaging can't compete price wise with the mass produced pellets in .22 cal.... For the foreseeable future, I think bullets will remain practical only at the upper range of weights for a given caliber, in the range of twice what the common pellets weigh.... That means 35-45 gr. in .22 cal and with the new 34 gr. JSB coming out in .25 cal, likely in the 45 gr. and up weights in .25 cal.... Really powerful PCPs only need apply....

Bob

[HPAJunkie]

I wonder if a hollow bullet could be swaged then reswaged to give it a hollow chamber inside. Or if there were some sort of polymer inserted to fill the cavity then swaged in order to give it the desired shape. That way weight is down but profile is more boattail for better BC than diablo pellets.

[Bwalton]

Boy am I late on this topic......let me read all the pages and see what I can add.

[Scott Endler]

The .177 JSB heavies have a much less pronounced waist similar to the photo than other pellets and a BC of .0310 in Chairgun compared to the  JSB Exact at .0210.
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https://www.pyramydair.com/product/jsb-diabolo-exact-heavy-177-cal-10-34-grains-domed-500ct?p=388
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But fit really tight and don't group as well at the 12-20 yards that I usually shoot in the yard as the Crosman lights in either of my .177 Walther barrels.