GTA
General Discussion To Gateway To Airguns => Airgun Gate => : longislandhunter April 21, 2010, 04:34:05 AM
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I am the first to admit that my math skills are basic at best so the necessary calculations to answer my question are far beyond my mathematical abilities. I could probably answer my question by the "trial and error" testing method but having been impressed many times with the skills of our members I figured it would just be better to ask the question and let those with the knowledge give me an answer....... Okay, here's my question...........
It's generally accepted that it's best to keep the speed of pellets under that magic 1100 fps mark due to the fact that passing 1100 fps pellets tend to lose the aerodynamic qualities of flight that are needed for accuracy. Powder burners of course don't suffer from this phenomenon and I'm assuming that fact is due to the power/weight ratio of the projectiles. So, my question is with respect to air rifles what weight pellets would be required in order to "fly well" at speeds well over the 1100 fps mark ?????
The reason I ask is simply because there are some PCP rifles out there that are fully capable of easily pushing a pellet over the 100 fps limit, such as my Condor, and it got me to wondering........
Perhaps my question really doesn't even make any sense,,, if this is the case then just let me know,,, my feelings won't get hurt :)
Jeff
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Good question ? Like to hear some answers to this. :emoticon:
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It is more the shape of pellets that cause them to destabilize when they break the sound barrier than any other factor. If you wanted to break the sound barrier and still keep some accuracy I would try the Promethius Pilediver.
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I Think pellets become unstable at high speeds primarily because of their shape.
Consider the 17-HMR bullet. It is basically a 17gr pellet that has a ballistic efficient shape, and travels at 2600fps.
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Jeff,
it's not that a pellet loses its ability to fly straight at supersonic speeds. It's that it's unable to sustain that speed; the loss of accuracy occurs at the transition from supersonic to subsonic speeds.
In fact, a pellet flying at supersonic speed is plenty accurate. Here's a simple version of what happens when a projectile goes from supersonic to subsonic. When something is traveling at supersonic speed the sound radiating from it "piles up" in a cone behind it- a shockwave. When it happens with a bullet we hear it as a "crack" sound; when an airplane does it we call it a sonic boom. As a pellet drops below supersonic speed the shockwave it built up catches up and overtakes it, buffeting it in the process. In addition there's a range of velocity overlapping the subsonic/supersonic transition; something flying in this range is said to be transonic" as some (but not all) all of the airstream across its surfaces is going supersonic relative to it. Having only some of the airstream across an object's surface going supersonic relative to that object results in unpredictable instabilities. I guess what you should take from this is that
1) a properly stabilized projectile traveling at subsonic speed will do fine
2) a properly stabilized projectile traveling at supersonic speed will do fine
3) a projectile traveling at transonic speed or that transitions from supersonic to subsonic speed will suffer some degradation in accuracy
So to keep a pellet accurate when fired at a supersonic muzzle velocity we need to alter its properties so that it stays supersonic. And it's not just pellet mass that matters for sustaining supersonic flight. It's the pellet's ballistic coefficient (in which its mass is a factor). The higher the BC, the better a projectile's ability to maintain velocity. Unfortunately, most airgun pellets are deliberately designed to have a low ballistic coefficient.
Pellets are designed to be stable at subsonic speeds even if they don't have much spin. This is why they have their distinctive diabolo shape- a head narrowing to a waist then a flared out skirt. The flared skirt is a design which deliberately creates a lot of drag on the end of the pellet, helping it to resist changes in its orientation (and therefore trajectory). It operates on the same principle as the shuttlecock you use to play badminton.
Unfortunately, just as in the case of the shuttlecock, this relatively high amount of drag slows pellets down much faster than the bullets that powderburners use. And therein lies the problem. You can shoot a pellet at supersonic velocity, but its abysmal ballistic coefficient means it will slow down to transonic speeds very quickly and experience the negative effects I described above. To give you an idea of just how bad the disparity is, here are some ballistic coefficients (higher BC means projectile will retain velocity better):
7.0 gr .177 RWS Hobby- .011
11.9 gr .22 RWS Hobby- .010
10.5 gr .177 Crosman Premier Heavy- .026
14.3 gr .22 Crosman Premier- .026
100 gr .243 Win spitzer point, Boat tail- .403
165 gr .308 Win spitzer point, Boat Tail- .477
even a 40 gr .22lr hollowpoint (which doesn't have a reputation for being terribly aerodynamic) can have a BC on the sunny side of .140
So the problem really is that you can send a pellet out of the muzzle at an arbitrarily high velocity, but it's going to slow down to subsonic speeds in one hell of a hurry, and as it makes the transition it'll lose some accuracy. Yes, you can hammer out a pellet really fast, but I doubt you'll gain much in the way of accurate range.
Cruising on over to SraightShooters and looking at their numbers for the Eun Jin Sumatra:
.20 cal Beeman Laser (BC .016): muzzle velocity: 1219 fps. @10 yrds: 1033 fps
.20 cal Beeman Crow Magnum (BC .024): muzzle: 1181 fps @10 yds: 1057 fps
the Laser, with its more or less average BC, in only 10 yds, hemorrhaged nearly 200 fps and became subsonic
the Crow Magnum, with its high (for a pellet) BC also, in only 10 yds, already dropped over 100 fps and became subsonic
Okay, so diabolo pellets suck at maintaining velocity. So what about a pellet shaped more like a bullet? Luckily StraightShooters has the BS Cylindrical pellet listed:
.20 BS Cylindrical (BC .023): muzzle: 1132 fps @10 yds 1022 fps.
ouch, the non-diabolo pellet didn't fare so well either. No big surprise when you see its BC, though I must admit I'm surprised its BC is no better than that of the Crow Magnum.
So really, what you need to do to a pellet to give it a better ballisitic coefficient is...
...make it into a bullet. Make it longer, heavier, sleeker. But here again there is a problem. Without that flared skirt, we lose some stability. So how do we make up that stability? We increase it gyroscopically by making it spin faster. But the longer and skinnier the projectile, the higher the spin required. This makes me wonder whether the barrel of a condor (or most any airgun, for that matter) has the proper twist rate to stabilize our hypothetical high-BC pellet.
Now, note that accuracy is relative. Some PCPs have decent accuracy out to 80, 90 yards right? But I'm guessing they're doing that with a big, heavy, fast spinning subsonic pellet. Heck maybe the pellet is supersonic at the muzzle. I dunno. But it's still a big, heavy one lol. I don't think there's any way in heck that the pellet is supersonic for very long, however. Just stabilized very well so that it tolerates transonic speeds well.
But to finally get around to answering your question (and some unasked but equally important ones): "So, my question is with respect to air rifles what weight pellets would be required in order to "fly well" at speeds well over the 1100 fps mark ?????"
Well, they're going to have to be heavy. .22lr heavy. And the gun's barrel will need to have an appropriate rate of twist for that heavy pellet. And they're going to have to be .22lr fast. And the PCP shooting will have to be able to produce that kind of power.
--> And, most importantly, you're going to need a pellet with a stellar ballistic coefficient. I don't know if any of the solid/cylindrical ones fit the bill, but it's certainly worth looking into.
Hope that was coherent enough to help,
Eric
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Eric, usually my ADD kicks in when I see a long post, but I read yours thoroughly and enjoyed it. Thank you for sharing your knowledge
Chris
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Chris, lemme tell ya, my ADD made it pretty hard to finish the thing lol. Glad you appreciated my unfocused ramblings (http://../jscripts/tiny_mce/plugins/emotions/images/smiley-laughing.gif)
Eric
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I am once again very impressed with the knowledge of our members !!!!!
Thank you all very much for the information you provided, especially Eric for putting such effort into your response and wording it in such a way as to give me a good mental picture of what you were saying :)
Thank you all again, I have my answer and I truly enjoyed the information you guys passed on to me !! :)
Jeff
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Thanks for the interesting topic Jeff. And thank you Eric for the detailled insight. Very interesting. I was going to suggest the BS cylindrical pellets as well,..not to mention that I was surprised with the results SS's came up with. I figured the higher BC would have made a bigger difference. But like you said, there are too many other requirements EX. maintaning the proper speed and spin of the pellet etc to remain stable in flight. Really puts the high velocity claims AG makers tout into perspective. Excellent work Sir, tjk
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All the new air Gunner's out here need as much information as possible. I have learned more on this forum than I could have hoped too!! The older I get the more I realize how much I don't know. Thanks for sharing.
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Eric,
that was the finest explanation of ballistic efficiency I have ever read. Thanks very much for the informative post.
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Eric that was the most comprehensive and yet understandable way I have ever seen anyone answer a question. I love ballistics and have a lot of trouble explaining this to friends and you did such a great job I think this thread should be posted in the Library. Just my two cents
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Great work on that Eric!
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I am playing around . And I am slow and I am waiting on a resizer now for my .25 caliber Condor bullets . But they should look like these .357 some day .Marvin
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New guy here!
I actually asked this question to a guy (Chris) that works for Sierra bullets. His answer was a little less complicated. He told me that pellets and rimfire ammo share the same problem. Both are much more accurate at subsonic levels simply because lead starts to deform at 1200fps, thus degrading accuracy. He also pointed out that the little 17Hm2 has no problems with outstanding accuracy and it weighs 17grains and travels at 2100fps.
Hope that answer helps a little.
Daniel
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Pellets aren't the only "wasp waisted" projectiles that suffer from instability.
About 12 years ago a friend and I did some reloading and testing of Lyman wasp wasited 12 and 20 ga saboted slugs in both rifled and smooth bore shotguns. Accuracy was miserable.
Also, some of the early rifled barrel shotgun sabots such as those from Winchester and Federal were long, wasp waisted slugs and had accuracy/stability problems. With many rifled shotguns, they would actually turn sideways and "key-hole" the target out beyond 50-60 yards. Those designs were dropped for more bullet like sabots such as the copper solids, which are much more stabile at long range.
Paul.
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WOW! That was pure poetry! I have a tear..Beautiful!
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um wow thanks for the praise folks. I wrote it right after a chemistry test; I guess maybe I was still in 'focused mode' lol.
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That was very spot on Eric!
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Really great post Eric!!!!
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The short answer - The diabolo (diablo?) or shuttle-cock pellet does not do well over 1100 fps due to the transitional forces breaking and then falling under the speed of sound. Simple as that. PCP's shoot well at super sonic speeds because they are shooting more traditional-shaped bullets. A spitzer (pointed) boat tailed bullet will be very accurate over 1100fps.
As to the Sierra man, lead does not deform at 1200 fps. Why would it? You can shoot a lead bullet into test media and recover it, intact, with no apparent damage. If you shoot a soft lead bullet over a heavy charge without a gas check on the base, it might melt the bottom, but that's from the powder, not the air.
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RedFeather - 4/22/2010 3:38 PM
As to the Sierra man, lead does not deform at 1200 fps. Why would it? You can shoot a lead bullet into test media and recover it, intact, with no apparent damage. If you shoot a soft lead bullet over a heavy charge without a gas check on the base, it might melt the bottom, but that's from the powder, not the air.
I never thought about the why's of his answer. I assume that Sierra bullets has performed testing that led to this answer. Otherwise why would he tell me this? I'm by no means a Ballistics guru, so when I have questions I call or email bullet makers for my answers.
Daniel
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Dan,
Not to doubt you, or put you on the spot, but I also wondered about that Sierra man's statement of lead distorting at over 1200fps .
As you may know, there are quite a few 22 rimfire non-jacketed lead, high velocity rounds that seem to do well at over 1200fps.
I would be interested to know more if you get to talk to him again.
Paul.
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He might be referring to the pellet skirt getting blown out which I suppose is a possibility on some thin walled pellets! Anyway, nice description Eric and Red for putting it in a nut shell!