 |  | | | |  | |  |  | 950 fps - The Magic Number |  | |  |  | |
 Posted: Wed Jan 25, 2012 12:11 pm |
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| rsterne |
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Just a few thoughts based on my experience and measurements....
1. As you push velocities into the transonic region (over Mach 0.8-0.85 ie 900-950 fps) accuracy often suffers due to buffeting....
2. As you push velocities into the transonic region the Ballistics Coefficient (and hence the wind drift) suffers....
3. At a velocity of 950 fps, the FPE is double the pellet weight.... Unimportant, but it makes for an easy calculation....
I use 950 fps as my "speed limit".... If the gun is capable of exceeding that, I change to a heavier pellet.... and ultimately a larger caliber....
Bob |
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Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
Airsonal: Too many to count! |
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| Posted: Wed Jan 25, 2012 12:46 pm |
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| AirGunEric |
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There goes Bob, bragging about his high-power creations again... 
No, seriously- why is buffeting starting at mach 0.8 instead of right at/much closer to mach 1? |
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| Posted: Wed Jan 25, 2012 1:06 pm |
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| Slavia |
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Because that nice conical shock wave doesn't appear by magic at exactly mach 1. (Formed because the air can't get out of the way and "bunches up.") It forms and dissipates randomly at speeds leading up to mach 1 (the transonic region). Hence the buffeting.
I was awake that day in class. Bob might say he likes subsonic, but I'll bet he's working on a way to keep pellets solidly supersonic for their whole flight. |
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 ¡Listo! ¡Apunte! ¡Fuego! |
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| Posted: Wed Jan 25, 2012 1:12 pm |
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| rsterne |
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While the entire pellet isn't supersonic until Mach 1.0, the locallized airflow over curved sections and edges can be supersonic and create locallized shock waves quite a bit below Mach 1.0.... If you have ever seen a photo of an F-18 doing a high "G" manouver at an airshow with a distinct vapour cloud covering the strakes in front of the air intakes, you are seeing that shock wave in action.... The plane is only doing Mach 0.8 or so, but the air flowing over the strakes is going critical and shaking the water vapour right out of the air.... The same thing happens on a pellet as it enters the transonic region.... and those shockwaves can disturb the flight....
Bob |
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Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
Airsonal: Too many to count! |
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| Posted: Wed Jan 25, 2012 1:36 pm |
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| AirGunEric |
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| Has anyone ever determined where, specifically, on a diabolo-style pellet where these ramp-ups in airflow/speed are occurring (i.e. head, tail, fins in tail), and tried to design a better pellet for it, or are we left with the standard "bullet" shape beyond about mach 0.8? |
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| Posted: Wed Jan 25, 2012 1:57 pm |
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| rsterne |
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The two logical places for a locallized shockwave to form are near the back edge of the head at the largest diameter.... and at the back of the skirt (again at the largest diameter).... I have never seen any studies published about it, however, that's just from theory....
A true round-nosed pellet like a JSB Exact would have a smoother transition at the back of the head than a "domed" pellet like an RWS SuperDome, and much smoother than a pointed pellet or wadcutter.... That "sharp corner" not only causes a lower BC, but is the perfect place for a locallized shockwave to form.... I believe (no proof, however) that is the reason that the Exacts (and a few others, the Baracuda/Kodiak, the FTT/FTS, and the Premiers) not only have a better BC than most, but tend to produce tighter groups at long ranges, and especially when pushed fast.... They simply have a more aerodynamic shape....
The new JSB Monsters (13.4 gr. in .177 and 25.4 gr. in .22 cal) have a much larger "waist" than other pellets, and so, in theory, should have less chance for "separation bubbles" and "locallized shockwaves" to form behind the head.... So far, although their accuracy is pretty good, I haven't seen them perform better in any of my guns.... Perhaps they are best for the "uber-magnums" where the Exact Heavies would be pushed at 1000 fps or more and the only way to tame them is with the Monsters....
I have only tried one "bullet shaped" pellet, and that is the H&N Rabbit Manums in both .177 and .22 cals.... They have been a complete and total disaster in every gun I have tried them in.... Maybe if you can push them well past Mach 1 they might be OK.... but so far, for me, they are a waste of money....
Bob |
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Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
Airsonal: Too many to count! |
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| Posted: Wed Jan 25, 2012 2:17 pm |
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| oddtodd |
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| Then why are all the manufacturers trying to break mach one? Wouldn't that make their rifles act poorly? I know my storm only gets to about 850 fps, but it's advertised as a 1000 fps gun. Are they purposely lying to us, or are these speeds actually obtainable? |
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Airguns: Two starlings talking "Larry, did you hear that? Larry?? LARRY!!!!" |
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| Posted: Wed Jan 25, 2012 2:19 pm |
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| BBgunNoob |
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| BOB why are you on this forum and not in a physic's lab working for NASA |
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| Posted: Wed Jan 25, 2012 2:23 pm |
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| Slavia |
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| Quote: |
| H&N Rabbit Manums in both .177 and .22 cals.... They have been a complete and total disaster in every gun I have tried them in |
Maybe because the center of mass is back closer to the center of lateral pressure, so drag stabilization isn't as effective - plus not enough spin to compensate? |
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¡Listo! ¡Apunte! ¡Fuego! |
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| Posted: Wed Jan 25, 2012 2:23 pm |
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| oddtodd |
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| BBgunNoob wrote: |
| BOB why are you on this forum and not in a physic's lab working for NASA |
Nasa is a side hobby for him.  |
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Airguns: Two starlings talking "Larry, did you hear that? Larry?? LARRY!!!!" |
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| Posted: Wed Jan 25, 2012 3:38 pm |
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| AirGunEric |
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| oddtodd wrote: |
| Then why are all the manufacturers trying to break mach one? Wouldn't that make their rifles act poorly? I know my storm only gets to about 850 fps, but it's advertised as a 1000 fps gun. Are they purposely lying to us, or are these speeds actually obtainable? |
Obtainable on some guns, sure. Why the "big numbers"? Because Gamo proved (they were the first to do it) that "speed" sells- even if the published numbers border on "false advertising".
As for Bob and NASA- he's screwed- I'm thinking there are more than a couple large-scale magnetic fields in their research facilities- he'll have to stick with being a mole running around the lower levels of a missile silo. Just kidding- Bob doesn't work at the bottom of a missile silo (or does he?). |
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| Posted: Wed Jan 25, 2012 7:11 pm |
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| radar |
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| AirGunEric wrote: |
| ...Why the "big numbers"? Because Gamo proved (they were the first to do it) that "speed" sells- even if the published numbers border on "false advertising"... |
I got that some years ago when I first shoot my Shadow 1000 through a chronograph and got less than 850 fps with a mid-weight lead pellet.
Just like oil filters and similar products. "99% Efficient!!!"
(= Completely meaningless without test particle size and SAE or ISO test type given.)
| rsterne wrote: |
| ...The new JSB Monsters (13.4 gr. in .177 and 25.4 gr. in .22 cal) have a much larger "waist" than other pellets, and so, in theory, should have less chance for "separation bubbles" and "locallized shockwaves" to form behind the head.... So far, although their accuracy is pretty good, I haven't seen them perform better in any of my guns.... [...] |
They are neither better nor worse in accuracy in either my Discovery or RWS 34 than most JSBs (which is very good.). They should extremely good at deep penetration in soft tissue, however, for hunting. No better, if as good through hard material though, as that seems to prefer higher velocity and foot pounds of KE more than slow moving momentum. At least this is true of the RWS 34, as the weight is "over-the-top" in power efficiency for that particular rifle. |
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| Posted: Thu Jan 26, 2012 2:33 am |
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| BBgunNoob |
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| AirGunEric wrote: |
As for Bob and NASA- he's screwed- I'm thinking there are more than a couple large-scale magnetic fields in their research facilities- he'll have to stick with being a mole running around the lower levels of a missile silo. Just kidding- Bob doesn't work at the bottom of a missile silo (or does he?). |
Is that why he lives in the middle of nowhere, bob are you secretly protecting a missile silo |
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| Posted: Thu Jan 26, 2012 3:02 am |
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| NZ Shooter |
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Cool, I wondered why my Patriot is so darn predictable, and consistant.
It spits a consistant 890-903 fps. The pellet flight is so dependable in any conditions. Except rain. I'd cry if My girl got wet. 
That last comment could be taken differently........  |
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| Posted: Thu Jan 26, 2012 8:54 pm |
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| Mark5043 |
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| AirGunEric wrote: |
| oddtodd wrote: |
| Then why are all the manufacturers trying to break mach one? Wouldn't that make their rifles act poorly? I know my storm only gets to about 850 fps, but it's advertised as a 1000 fps gun. Are they purposely lying to us, or are these speeds actually obtainable? |
Obtainable on some guns, sure. Why the "big numbers"? Because Gamo proved (they were the first to do it) that "speed" sells- even if the published numbers border on "false advertising".
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Speed does sell....it's quite common to equate speed with power,without factoring in pellet weight,and overall energy..accuracy,and pellet stability on the way to the target. |
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