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It's the "Pellet head shape/size influences long range accuracy thread  

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Harvey
(@harvey)
Joined: 2 years ago
Posts: 103
2020-05-23 08:37:53  

I know this topic can get convoluted and possibly ugly.  There are so many variants which could affect (and probably do) the end result of my question we might not conclude a likely answer.

Do you think there's a correlation between pellet length and longer distance accuracy?

Would velocity and rotation speed play into it.  I think they would but I'm interested in what you all think.

Longer pellets?  Stubbier pellets?

Harv

 


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awilde
(@alex-from-upstate-ny)
Joined: 2 years ago
Posts: 162
2020-05-23 12:07:21  

I think the shape and weight of the pellet tends to come more into play than anything else in the big scheme of things. That being said I also tend to find that every gun (be it airgun or firearm) will generally have specific projectiles/loads/configurations that it will like for the best range and accuracy, even within models of the same gun.

An example shape wise tends to be stuff like wadcutters vs domed, where you have significant differences in both shape and weight (and lengths depending, some WC's I've seen with longer skirts and then others are very short).

Length I can't really say one way or another because I've bought a bunch of different tins of pellets that have lengths all over the place from super short to almost too long (to the point where they barely can fit into a multi-round magazine) and had distances and accuracy all over the place.

I think one of the difficulties is simply going to be finding the sweet spot on any given airgun for distance and accuracy with a given pellet. On the couple PCP's I have there are wildly different regulator/hammer/xfer port settings that pull in different types of pellets from barely being able to hold a 6 inch group to punching out dime and quarter sized groups at distances from 25-50 yards (nothing I currently have has been able to do much past about 60 yards though without opening up too much). Finding something that will work across multiple pellets with the same configuration hasn't been an experience I've yet to encounter on anything I have unfortunately, but I'll be the first one to admit I haven't had the luxury of being able to afford any of the more schmaltzy and expensive airguns with my budget usually in the $250 and less range.


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straitflite
(@straitflite)
Joined: 2 years ago
Posts: 176
2020-05-23 13:00:50  

My thoughts lean toward fit and balance, especially a pellet that would be truly balanced. I doubt a device exists for checking that, to the consumer within reasonable cost.

Convoluted? Science cannot be anything but 😀 


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270windude
(@270windude)
Joined: 3 weeks ago
Posts: 26
2020-05-23 13:52:57  

Heavy for caliber bullets will be longer. Their greater BC and SD will provide better “long range accuracy”. If you machined a long pellet without mass it would not fly straight. Mass and shape imo are the characteristics to look at. This might convolute any correlation you’d notice between length and long range accuracy  

At 30 yards I’ve had many guns shoot the short little JSB RS on point (almost every springer I’ve shot) yet heavier pellets fly all over. Obviously every gun will shoot different due to its own harmonics, barrel idiosyncrasies, etc. 


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Cruisers
(@cruisers)
Joined: 2 years ago
Posts: 82
2020-05-23 20:52:55  

Yes I do believe longer vs fatter would be more stable at distance. Many years ago thinking about Doc Beemans writings on the .20 cal started me on this train of thought. Then I got my first .20 cal, a Rapid. It hit me that the .22 Premier was exactly the same weight but the .20 was slimmer and longer. To this day of the "basic" calibers, .177 .20 .22 and .25, my old .20 Rapid is still the most consistently accurate airgun I've ever shot beyond 35 yards. I've always wondered if the slim long tail was the reason for this.


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270windude
(@270windude)
Joined: 3 weeks ago
Posts: 26
2020-05-24 01:45:05  

It’s not JUST because the pellet is longer imo. Just found the right pellet for the gun. Sectional density is what’s at play. We don’t hear much ballistics talk on airgun forums but on benchrest rifle forums it’s old news now.  I’d recommend reading some of Brian Litz’s Books. I’ve followed him a bit and read modern advancements in long range shooting volume 1 and 2. Excellent info. Some of the info transfers and some will at least get you thinking...


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James Perotti
(@jpsaxnc)
Joined: 2 years ago
Posts: 86
2020-05-24 07:38:57  

A pellet with more surface area, (longer or fatter) will be more affected by drag at any distance. I think accuracy is a seperat issue though.


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Hector J Medina G
(@hector-j-medina-g)
Member of Trade
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Posts: 391
2020-05-24 09:58:45  

Pellet ballistics are hard to retionalize because we do not, generally, understand them well.

A lot of shooters will say that pellets are "drag stabilized" Like badmington shuttle cocks. Not always true.

Some will say they are spin stabilized, like most projectiles flying out of rifled barrels. Not always true.

Some think they are aerodynamically stabilized, like arrows. Not always true either.

For the MOST part, pellets are "flange stabilized" meaning that the head creates a "wind tunnel" where the skirt travels. If the skirt moves out of its axis, it hits the tunnel wall and comes back to center (at the price of loss of velocity).

The relation between the nose, angles and curvature of the head determine the length of the tunnel opening. If it is too short, then the skirt will be travelling in a closing gap, velocity will  suffer and drag will set in. Loss of velocity and turbulence come into effect and instabililty drives the pellet off course.

The size and shape of the tunnel is also affected by the rotation and surface treatment of the head. Dimples and grooves that rotate open up the tunnel, enabling  that pellet to travel better and further.

But rotation also brings in a factor: at extreme speeds, it will make the pellet unstable, and therefore the "corkscrewing" of the skirt will continually hit the tunnel walls and end up in a larger drag, lower BC, and overall worse performance.

The last aspect to all this is that we now love to shoot our pellets at near trans-sonic speeds. Back in the days 500 fps was considered good enough for supreme accuracy, at 10 meters, we now want to shoot with the same accuracy out to 50 meters, BUT, air density and viscosity have not changed, so our aerodynamical limit is still about 975 fps give or take 25 fps for a waisted pellet.

There are EXCELLENT trans-sonic/ sub-sonic designs for slugs, but the truth is that they do not look "sexy" and it's hard to make them shoot well in springers.

On the other hand, the LOUSY BC is what makes airgunning a backyard-possible form of shooting. Without that really bad BC, the safety zones would be 3 or 5 times larger.

So, to answer the question: Longer pellets WITH THE RIGHT head shape ARE better for the long range game, as long as they are light enough to be driven to the flange-stabilized portion of the external ballistics aerodynamics, but not too fast as to infringe on the stability limits. And that has to fit within the power restrictions of some games and/or countries.

And we still  have not begun to discuss the effect of the weight distribution on the efficiency of the pellet itself. . . . 

Anyway, keep well and shoot straight!

 

 

 

 

 

HM


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dcw
 dcw
(@dcw)
Joined: 2 years ago
Posts: 72
2020-05-24 16:31:13  

from what I've observed  thru the years, round/domed pellets seem to perform at longer distances better than others...lots does come into play, though...speed...weight...distance..., rotation...shape...nose design...it usually ends up with experimenting for best performance in your application... 😎 


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270windude
(@270windude)
Joined: 3 weeks ago
Posts: 26
2020-05-24 16:35:40  

I’ll attempt devils advocate here for fun...  applying it in real terms isn’t as simple as theory suggests imo. A short jsb RS has shot better at 40 yards then almost any longer pellet(not just longer but the “long for caliber”) for me across an array of guns: pile driver, jsb heavies (not jumbo).

Hector did great pointing to what makes a pellet fly straight (obviously one of the top knowledgeable guys) but I still hold that there are too many variables to blatantly say longer pellets are better at range which is why I’d put extra emphasis on his caveats towards the end. If I were designing my own 50 yard spring we pellet I’d probably consider weight and shape first rather then just building around what’s considered a long for caliber. Do longer pellets have ballistic advantages provided they maintain requirements?  Ya I’ll agree with that. Just like in center fire though, higher BC certainly does not equate to higher precision. 

Also... I would at some point like to learn more and discuss weight distribution! It’s been on my mind recently. I understand centerfire ballistics more then most I’d say but am just learning airguns and have lots of questions...  I’ve had bad luck with the shorter head domes. Comparison RWS Superdomes VS JSB Exacts. The more flattened heads haven’t shot well in my (admittedly lower end compared to what others shoot) springers. 


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Cruisers
(@cruisers)
Joined: 2 years ago
Posts: 82
2020-05-24 19:51:02  

Hector, thanks for the info.It brings another question to mind. IF, the pellet is not hitting the inside of the moderator, what is the most likely reason for it to hit the target sideways. (Late model BSA PCP going around 850 fps .22 w/JSB different weights). Not all shots do this, but maybe 2-3 out of 10.


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Harvey
(@harvey)
Joined: 2 years ago
Posts: 103
2020-05-24 21:53:40  

Wow!  This turned into an awesome thread!

Hector, If I understand correctly this could be why the old Kodiak pellets shot so well in any of my rifles which could handle them.  I think someone tried to come up with a benchmark in that if a rifle shot at a certain velocity with say, a 14.3 grain, and the Kodiaks were... I forget what they were, but if they worked well enough we could expect better long range accuracy

The length of the head of the pellet in relation to over all length?  Or am I on the wrong track?

I keep envisioning a "taller" dome where the limit becomes the point where the peak loses its eccentricity to the diameter.

Harv

 

It's like spitting apple seeds when you just ate an orange


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Hector J Medina G
(@hector-j-medina-g)
Member of Trade
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Posts: 391
2020-05-25 10:40:15  

@ DCW.- Yup, lots of things need to converge to get that supreme pellet (and it will, most probably, be supreme in some platforms, but not others). One of the REAL problems with pellets is that the dies they are made in, do wear out and so, new dies need to be made, BUT how to replicated EXACTLY the shape of the "good" ones?Many companies are looking at this problem and the best solution has been to use a "master" steel pellet to then carve a copy out of graphite that is used to EDM the cavity on the dies. Also modern inspection methods are coming on line. As much as we complain about the current pellets (sometimes we gripe mostly about price), they ARE head and shoulders above the offerings of old.

@ 270WD.- The RS's are great pellets in smallish guns. Where other pellets quoted would not have the chance of reaching a stabilizing speed due to weight and/or shape, the RS's easily would have done so. It is all in the BALANCE of all aspects, just enough speed (linear as well as rotational), alignment to the bore's axis, speed of rifling twist, AND something that is seldom considered as much as I try to emphasize it: The LAST SWAGE DIE in the life of the pellet is the specific barrel it is shot from. There is no escaping this. SOMETIMES, I've seen RS's yield phenomenal accuracy at 40 yards, only to open up like a shot-gun pattern at 55, and/or with a whiff of a wind. Others I've seen pellets that have been touted as having EXCELLENT accuracy at 1,100 fps only to recover some and witness that the pellet's waist is gone, and what is coming out of the barrel is a slug. In pellet design the important relation is the relative location of the center of pressure in relation to the center of gravity. MOST of the times, the CoP is located BEHIND of the CoG at subsonic speeds, but moves forward as speed is increased much faster than what would have been expected. So, a pellet will behave in DIFFERENT manners at the short, medium and long ranges.

@ Cruisers.- USUALLY, BSA barrels are very good, hammer forged and conical (not choked), they can be VERY accurate. It's the only aspect that GAMO has never even dared touch after purchasing BSA more than 2 decades ago, so that tells you how much respect those barrels command. To have a pellet destabilizing enough to keyhole, is a big warning sign! I would remove all shrouds, baffles and encumbrances, and shoot some pellets at 50 yards/meters, if still 2 or 3 out of 10 keyhole, then I would send a nice  letter to BSA explaining what I had found. I would assume they would send you a new barrel. It's important NOT TO TOUCH the barrel more than needed. Just remove all accessories and make sure that you are analyzing what the barrel, BARE BARREL, is doing. I would also ensure that the MV of the pellets is uniform and consistent all through the tests. Reasons could be several, but it is more important now that you establish IF the barrel is at fault, or if there is something else that is going "funky".

@ Harvey.- Baracuda/Kodiaks were 21.2 grs. in 0.22" and they WERE, for a time, the best long range pellets out there. Then along came the JSB Jumbo. At only 16 (15.9) grs, they achieved stability velocity with much more ease than the heavier Baracudas. Until recently, no other pellet had held as good a long range performance on record (out to 200/250 meters) as the Jumbo Exacts. As airguns have become more and more powerful, the larger 0.22" pellets are proving to have a better performance record.
And there are many factors to this, the SHAPE of the head is more important than the relative dimensions. RWS superdomes have a shorter than hemispherical head, Baracudas, have a secant elipse head, JSB have a parabolic dome head. Each shape has its advantages and disadvantages, but if you get the BALANCE between CoG and CoP right, the BEST head is the secant elipse one. This is the head shape that, at sub-sonic/slightly trans-sonic regions creates the longer "tunnel" and therefore allows a longer Time of Flight for the pellet to reach its destination without undue interference. You just need to ensure that your pellet is still light enough to achieve stability at reasonable power levels.

Of course, if the MECHANICAL fit isn't there, no geometry will overcome that limitation.

As much as we want to discuss this, the truth is that we each have to "let the gun talk for herself".

 

😉

 

Have a meaningful and thoughtful Decoration Day (as enacted by Pres. Lincoln).

 

 

 

 

HM


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GLPalinkas
(@glpalinkas)
Joined: 2 years ago
Posts: 95
2020-05-25 13:12:01  
Posted by: @hector-j-medina-g

Pellet ballistics are hard to retionalize because we do not, generally, understand them well.

........................................ 

HM

Hector, As usual, I learn more from a 10 minute conversation with you or from a short forum post than I do in a month of reading other places. You are a true credit to, a great supporter of, and have a tremendous passion for the airgun community. I admire your "persistence" because it is easy to move away from a hobby but your continued support helps us all. 

Best safe my friend. Hope to see you on the lanes sometime this year.

Gary
Venice, FL


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GLPalinkas
(@glpalinkas)
Joined: 2 years ago
Posts: 95
2020-05-25 13:23:00  

ATTENTION FORUM OWNER - MODERATORS

As someone else suggested, this is a great thread. However, the title leaves most members out. I clicked on it by chance. Can a moderator re-title and make this a sticky? No offense to the OP. It's a great question but most won't bother clicking on that title.

Gary
Venice, FL


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270windude
(@270windude)
Joined: 3 weeks ago
Posts: 26
2020-05-25 15:18:22  

Hector, I completely agree with the RSs.  I try pushing them out but when they open, they open. But I don’t shoot beyond 40, that’s about my 1 mile shot at the moment. I just choose them sometimes out of necessity more then choice.  So from what you say, maybe my gun isn’t shooting fast enough to utilize the pellets design due to its CoG.  Hmm maybe that’s something I could start watching for and documenting, thanks  

There’s a lot that’s missed in airguns. In CF there’s open knowledge of various twist rates and gyroscopic stability, barrel length effects, lead angle, bullet seating in relation to rifiling, etc.  I recently saw on one thread were it was recommended NOT to seat pellets, that they are to be flush in the breech on springers. The same thread then says we have to seat them. Personally I seat my pellets, I’ve yet to begin seating them in relation to the rifiling but I will be using that as my seating variable soon, just as in center fire.

i found this list in another forum, it’s incomplete but provides some information regarding twist rates.  I’d also like to know how many “grooves” each has and any choking.  

TX200 1:17",
HW97 1:15.5" ,
Rapid Mk 1 1:14",
Steyr 110ft 1:17" ,
HW100 1:16",
CZ (S200) 1:18.5",
BSA 1:18.5",
Steyr barley twist 1:15.5"
HW50 1:16"

I would like to look at a pellet and say to myself “ah, this one should do well in a slower twist X groove barrel around 8fpe.  Common airgunner knowledge suggests that there’s less use to baseline information then there is simply trying a million pellets regardless of shape to see what your exact gun likes. Dreaming? It seems from my YouTube watching, that FX is doing a lot of experimenting with barrel design vs pellet design.

Pellets, variable overload... I feel we need a source that works to categorize the variables better.  Maybe there is but I don’t feel there is.  1) skirt thickness/ depth. Consider if it’s used in a magnum or low power. I’ve heard some high power guns, the air smacks so hard the pellet almost takes a more conical shape or deforms the diablo design as it presses to the barrel.  2) how hard is the pellet, the required amount of air to begin moving a harder pellet may have different recoiling characteristics or harmonics then a softer pellet. This may also effect velocity? If the pellet sits for a slightly longer time I’d suspect the PSI would be effected?  3) depth/interior shape of the skirt. We have short flat skirt interiors like with CP box and deep pointed/conical ones like RWS Superdomes. I’d like my guns in the 10-13fpe range to shoot simple looking and short air pockets. 4) weight distribution. Total loss here. Barrel length, twist rate, FPE, so much could come into play that I don’t even know where to start. 


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Harvey
(@harvey)
Joined: 2 years ago
Posts: 103
2020-05-25 18:23:29  
Posted by: @glpalinkas

ATTENTION FORUM OWNER - MODERATORS

As someone else suggested, this is a great thread. However, the title leaves most members out. I clicked on it by chance. Can a moderator re-title and make this a sticky? No offense to the OP. It's a great question but most won't bother clicking on that title.

Gary
Venice, FL

I changed it but if you come up with a better title, I can change it to that.

 


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crazyhorse
(@crazyhorse)
Joined: 2 years ago
Posts: 25
2020-05-25 19:24:13  

I was shooting 10,2 gr JSB's in my HPA 850 at 960 FPS and it would group consistantly at .625" over 8 shots at 51 yards. We then fit a custom made aluminum air stripper and after numerous groups/adjustments (gap) it settled in at .375". 


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270windude
(@270windude)
Joined: 3 weeks ago
Posts: 26
2020-05-26 16:18:16  

Barrel weights and strippers, oh my....  what’s the twist rate and groove type of that 850? 0:)  

After conducting some more research it appears that slower or fast twist barrels can stabilize the longer more bullet like pellets. Some custom benchrest barrels are like 1:26 shooting long pellets around 900fps which stabilize.  That is an insanely high RPM for a pellet IMO but it works for them I guess.  I’ve also heard much slower like 1:15 stabilizing them though. Lothar Walther making 1:16 or 1:17.7 for 177 and those twists come choked or unchoked. Crosman from what I gather is 1:15. So as standard consensus would suggest, gotta try a bunch of pellets.  I know that I always hope my heavier pellets will fly straight but at my effective ranges for springers the lighter ones almost always perform better sadly.  PCP is another animal all together  

I have a really hard time getting information on airgun barrels. Maybe if manufacturers could start publishing their twist rate, and type of rifling it would help us learn. 

But to remain somewhat on topic of pellets... I’d like to learn/hear more about weight distribution, and primarily skirt/air pocket (wrong term) design. Like I said I’ve had best lunch with smaller air pocket pellets. 


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Miles_M
(@miles_m)
Joined: 2 years ago
Posts: 42
2020-05-26 16:24:21  

As I worked exclusively in external ballistics and the aerodynamics of gun launched projectiles for 40 years, I intensively studied pellet aeroballistics in my spare time for the last 30 years using the same methods and systems I used on just about every other form of projectile. Pellets are aerogyro stabilised, that is they use both aerodynamic and gyroscopic stabilisation. The flare at the back of the pellet is what produces an aerodynamic stabilising moment about the CG. It does this by producing a lateral force, as explained by slender body theory, about two thirds of the way along the flare which produces the stabilising moment about the CG. This is why it is correct to refer to pellets as being flare stabilised. Note this is a lateral force due to the pellet yaw and the airflow around the pellet body. Any body shape will produce a lateral force as soon as there is a yaw angle to the airflow this includes pellets, bullets, shells, or house bricks no matter how small the angle. Stability is not produced by drag or any longitudinal forces as they do not have sufficient moment arm length between their line of action and the CG to create significant moments unless the yaw angles get very large in which case stability is the least of your worries. All types of stability are all about moments not forces. Hence pellets are not drag stabilised. The stabilising aerodynamiic moments produced by the diabolo pellet shape can only give relatively small stabilising moments, hence the requirement for some degree of gyroscopic stability as well.

In the design of pellets for long range one of the problems is the relatively large loss in forward velocity compared to the slow loss in spin rate coupled with poor dynamic stability properties. It does not really make a lot of difference if the pellet is long or short the key will be to get a well manufactured pellet with the right balance of aerodynamic and inertial properties which suits your gun barrel in order to minimise the initial yaw rates when the pellet leaves the barrel. In this way you are giving the dynamic instability inherrent in the pellet design nothing to work on in terms of yaw angles. A round nose shape will minimise nose drag but, since most of the pellet drag is base drag, will make little difference to the total once you have a reasonably round nose.

As for spin rates the important factor is the ratio between the spin rate and the forward velocity, this is what determines gyroscopic stability, not the absolute spin rate itself. The use of lower initial spin rates may be beneficial at long ranges due to the spin/velocity ratio not getting too high as the pellet slows down. This may improve dynamic stability. Dynamic stability is a very difficult parameter to predict being dependent on a number of aerodynamic coefficients including Magnus moments which are notoriously difficult to predict.


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270windude
(@270windude)
Joined: 3 weeks ago
Posts: 26
2020-05-26 16:49:21  

Wonderful post Miles!  I’m sure I can dig up some more question. Everything you said is now formed in my brain quite well as my opinions form...  Pretty pumped about the design being more important then just starting with a long pellet and designing around that, from my limited knowledge I completely agree. It’s interesting when choked barrels are brought into the picture. Would you suggest that this actually lessens the moments for stability or actually enhances on a, less then perfect, pellet design? Or is the choke more to create better uniformity?  If so there are gauges for that and sorting...  I’d like to think that the small change in CG from a choked barrel would benefit a larger percent of faster moving projectiles then slower. 


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crazyhorse
(@crazyhorse)
Joined: 2 years ago
Posts: 25
2020-05-26 21:55:03  

The 850 came stock with the Lothar Walther barrel and medium choke...twist rate is 1: 16. You may know that the stripper removes turbulance from behind the pellet (in this case) and that is why adjusting gap is important together with shooting groups.

 

 


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270windude
(@270windude)
Joined: 3 weeks ago
Posts: 26
2020-05-26 23:23:24  

Ok I was wondering. I thought that they had the LW barrel but wasn’t sure as I’ve seen conflicting information. Great gun regardless. 


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crazyhorse
(@crazyhorse)
Joined: 2 years ago
Posts: 25
2020-05-26 23:54:11  

@270windude

I've had mine since 2008. The HPA conversion really transformed these rifles.Not easy to tune the trigger though. At any rate, I believe adding the stripper also added some down range speed to the pellet.

My take on pellets:

Short range: Hollow points

Target & hunting :Round head

I never shoot pointed pellets but do shoot cylindrical's from my 20 cal pump guns.


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Hector J Medina G
(@hector-j-medina-g)
Member of Trade
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Posts: 391
2020-05-27 09:15:27  

I am glad Miles has put some order into the discussion. Though I am not sure I agree with the "slender body theory" thing. Basically because the Slender Body Theory ASSUMES that there is SOME flexibility to the body in question. And I cannot see a pellet as flexible body. Nor is it a "slender" one in reality. Fuselages and bodies of missiles, yes; some shells and bullets? Maybe. BUT Pellets? you would have to convince me of that.

IMHO the real problem is that we have MANY different conditions that apply along the trajectory of a humble pellet. And where and how those different conditions prevail over others depends on the final SWAGE DIE (the barrel). We are working at such low energy level that while the aerodynamic forces might prevail at some point, the gyroscopic forces can overtake the action in a few yards.

Yes it would be nice to be able to look at a pellet and say: "Hmm, this should shoot well in such and such barrels at such and such power levels" BUT I do think that we are seeing it in reverse. We have the pellets we have, we pick those pellets that are MOST CONSISTENT in manufacturing, materials and tolerances, and THEN we can decide at what speed and in what barrel to shoot them.

Shooting slugs may just be the "final solution". As the number of forces that act on a slug is simplified by the much simpler form of the slug itself. BUT, we will be loosing other aspects (such as backyard safety) that we do not seem to value much in these discussions.

Again. JMHO.

 

 

 

 

 

HM


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Miles_M
(@miles_m)
Joined: 2 years ago
Posts: 42
2020-05-27 14:30:20  
Posted by: @hector-j-medina-g

I am glad Miles has put some order into the discussion. Though I am not sure I agree with the "slender body theory" thing. Basically because the Slender Body Theory ASSUMES that there is SOME flexibility to the body in question. And I cannot see a pellet as flexible body. Nor is it a "slender" one in reality. Fuselages and bodies of missiles, yes; some shells and bullets? Maybe. BUT Pellets? you would have to convince me of that.

 

HM

Not sure what slender body theory you are thinking of but as I was taught it there was no mention of body flexibility in fact just the opposite. It mearly describes how the airflow creates lift around the parts of the body where the cross sectional area is changing based on Stokes flow and the Navier Stokes equations. It simplifies down to the fact that increasing area produces positive lift where as decreasing area produces negative lift. It describes why boat tails are destabilising, despite being behind the CG, as are nose shapes as they are infront of the CG.

The thing is it tends to work for just about any body shape except flat faced blunt bodies. It certainly works for flares and is the basis of much of the aerodynamic predictions for such shapes.


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