Large BC Differences — Due to Barrel Differences? 🤔

[JungleShooter]

Large BC Differences — Due to Barrel Differences? 🤔 

I just read elsewhere the following:

QUOTE:  "I've been surprised to experience how different barrels will have varying degrees of hold off necessary for similar wind values. All else being the same, i.e. same pellet at same velocity."



OK, I really don't like reading this. Really. Don't.
Sorry. 😖

(Thank you to the shooter to call attention to this issue, I have nothing against you!)


🔺 What I understand from this post is that
▪ the same pellet
▪ at the same MV
▪ from different barrels
➔ suffers from a LARGE difference in its ballistic coefficient.


🔸 I acknowledge there can be SMALL BC differences from different barrels.
But LARGE differences?
I don't like that.


🔹 Because it would make published* BC numbers pretty useless:
Every barrel—projectile combination would have to be tested. Individually.
You guessed it: I don't like that....
*(by airgunners, manufacturers, anybody)



😊 Please, I'd love a discussion on that.
And I am open to accept what I don't like.
Please, supply some arguments or some kind of evidence (drift numbers, BC tests, etc.).
Because the thing I like less than large BC differences is — to continue shooting in ignorance. 😉


Matthias

[subscriber]

I can see different twist rates affecting the vertical reaction to a side wind.

Else, even a "large difference" in choke diameter is not going to change the frontal area of a projectile that much.  So, the differences in BC due to a different projectile diameter after it exits the barrel probably is not very significant.

Differences in rifling "texture" engraved into a pellet or slug probably will have more effect on the vertical reaction in windy conditions, than down wind.

[K.O.]

.220

http://www.geoffrey-kolbe.com/cgi-bin/drag_working.cgi?unit_length=inches&weight_unit=grains&bullet_name=.338+air&re_calculate=yes&boundary_layer=L%2FT&entry=return&diameter=.220&length=.34&nose=.16&meplat=.08&drive_band=.220&base_diameter=.220&angle=0&boat_tail=0&secant_radius=&weight=31.6&density=11.4

Choked to .215 ; weight does not change but length does...

http://www.geoffrey-kolbe.com/cgi-bin/drag_working.cgi?unit_length=inches&weight_unit=grains&bullet_name=.338+air&re_calculate=yes&boundary_layer=L%2FT&entry=return&diameter=.215&length=.353&nose=.16&meplat=.08&drive_band=.215&base_diameter=.215&angle=0&boat_tail=0&secant_radius=&weight=31.7&density=11.4

[K.O.]

do keep in mind that this is only to two places... do not know how is rounded...

so..

longer choked round (sd .098) could conceivably be rounded down from .0649 & .749 ... shorter non choked round(sd .093) likewise could be rounded up from .055 on their B.C. ...that becomes a bit more significant difference... so sometimes figuring to 3 places will give a slightly better idea of what potentially is happening...

[K.O.]

then there is how  different shape slugs fly  they can wobble some  may wallow at distance...

what happens when you mix a long boat tail and a harmonics prob and higher residual pressure cause ya got the fpe with a skinny 24" barrel instead of a thicker/heavier 26"/28" barrel..? a Higher chance of yaw out of the muzzle..? 

[subscriber]

Thanks for running the calculations Kirby,

If I read that correctly, choking a .220 slug to .215 does not make much difference to the BC in the 0.5 to 1.0 Mach velocity range. 

A slug passing through a choke would get a little longer because the material has to go somewhere.  So, the form factor would also change a little.

While I used the word "projectile", Matthias was talking about pellets.  As pellets have narrow bands of contact with the barrel, choking a .22 pellet down by 0.005" in diameter would probably not elongate it significantly.

I agree that if a given barrel affected stability of the projectile; slug or pellet, such that it flew with more wobble or greater yaw angles, or with more time spent not tangent to the trajectory, that this should increase projectile drag, effectively reducing the BC.   Such a change in BC would apply also to the pellet flying in a spiral.  This is because not only would the projectile be flying with a more side-on aspect, the total distance travelled to target would be greater, than a similar pellet following a normal trajectory.

Having a ding on the crown that catches the pellet on one side as it leave the barrel muzzle could probably induce such a yawing moment with wobble, that effectively reduces the projectile BC.  A muzzle being cut off non-square may do the same thing via asymmetrical air blast, although a significant angle would probably be more obvious that a ding right on the edge of the crown.  A dished crown with the bore off center may have a similar effect - due to an asymmetrical air blast as the projectile "uncorks" from the muzzle.  If the crown is cut concentric to the barrel, rather than the bore.

Different barrels may mangle projectiles on loading, in such a way as to affect stability and thereby reduce effective BC.  For instance, if the TP has a sharp inner edge and scrapes one side of a the projectile.  Such a barrel may even group OK at a fixed distance, but may seem to perform more erratically when used on targets of opportunity at variable ranges.  Thus, disturbances in projectile flight may actually not be random; although the effect over variable ranges would seem like it.

Obviously, a barrel that mangles projectiles for obvious reasons could use some help.  Removing the causes for barrels mangling projectiles should tighten groups generally, whether the dispersion had been random or not.  Hence, this useful thread:  https://www.gatewaytoairguns.org/GTA/index.php?topic=130555.msg1288471#msg1288471

[rsterne]

There is significant evidence that different GUNS shoot the same pellet at different BC's.... Whether this is due to the barrel, or a different pressure pulse forming the skirt to a different shape is AFAIK unknown.... Certain more pressure can cause the skirt to expand to a different degree, and that results in a significant change of pellet shape.... Here is a photo from Lloyd Sikes showing 2 pellets pushed through a barrel, and then fired from the same barrel (pressure unknown)....



Obviously the obturation of the 2 pellets on the left from the pressure changes the shape quite a lot, and it doesn't take much imagination to think they would have a different BC....  Miles Morris did some calculations on the difference that wobbling would make to the BC, and found it should be significant, particularly when the yaw exceeds about 6 degrees.... At 10 degrees, he found a drop in BC from 0.029 to 0.023 with the pellet he was using in his calculations.... However, at the same time, that yaw cause a large increase in group size (reduction in accuracy).... You can read about this in Reply #57 in the following thread....

https://www.gatewaytoairguns.org/GTA/index.php?topic=169459.40

I would assume that no obturation occurs on slugs, and that would suggest that pressure effect may be less than with a pellet, but there is no question that with either, changing the velocity can change the BC because the drag models we are using to calculate it are not a great match for our projectiles....

Bob

[ballisticboy]

The first thing to look at is the validity of the statement "different barrels will have varying degrees of hold off necessary for similar wind values." Is this an observation under uncontrolled conditions or the result of scientific valid controlled trials? The first thing to notice is that it says similar wind values, not identical. How close is similar and is it similar near to the gun, all along the trajectory or at some discreet points? If the results of trials, how were the trials carried out, and how were the conditions measured?

If we assume for the moment the statement is true, then what could be the cause? One of the favourites is yaw but as I showed here https://www.gatewaytoairguns.org/GTA/index.php?topic=169459.msg156221666#msg156221666 if the yaw is big enough to give large changes in BC then you are looking at very large group sizes. Differences in barrel diameter is only going to give very small changes in BC, even if we consider the changes in pellet shape and SD due to squeezing the pellet to different diameters. There is a possibility that changes to the shape of the flare after firing from different guns can give changes to the base drag, but again I would not expect them to be huge. I know that the same projectile fired from different guns can have different drag laws. The same 155mm shells fired from different types of gun need different drag laws to match data, but we are talking of shells flying 25Km, not direct fire air rifle pellets. The same degree of change in drag law would give no change in BC at pellet ranges.

If the statement is to be tested, then it needs to be done in the correct manner. Two guns need to be fired either side by side or in interleaved serials to eliminate changes in wind conditions. All the atmospheric conditions need to be measured. The tests need to be carried out on a number of different occasions and all the results combined. I know someone in the UK tried to look at wind drift for two different pellets to compare them one to the other using two rifles fired at the same time side by side. Unfortunately, the results changed for the different occasions, so no conclusions could be drawn. The complete process needs very careful planning and execution.

Any volunteers?

[JungleShooter]

Thank you all for your input!! 👍🏼👍🏼 



Surely, this must be one of the deeper rabbit holes in airgunning....


🔸 For clarification:  The statement in a post that got me to ask this question referred to pellets — however I would not limit our discussion to pellets alone, slugs are just as interesting.



🔸 So, I understand that a barrel can be rather defective, and therefore deform the projectile in ways that will drastically reduce the BC.

➔ But, as mentioned, these same deformations will most likely also produce drastic loss of precision, so a shooter wouldn't even consider shooting this particular barrel, or that particular projectile from that barrel in a comparison or a competition.
🔸 Similarly, for yaw: It lowers the BC, but also the precision, so, really a mute point.


🔸 I can see how ​the engraving caused by the rifling might account for a small amount in performance.... 👍🏼

🔸 And more so, I can see how blowing out the pellet skirt (obturation) can have some affect on BC, as the whole shape of the pellet is changed. 👍🏼 I would expect a higher BC...?





🔸 Mile's prodding for the specifics of the comparison and their scientific validity — well, yeah, that's just what we're often missing in airgunning (and what Miles, Bob, and some others are trying to help with):

We have mountains of anecdotal "evidence" — and shreds of scientific evidence.

🔺 ​It really seems like a test would be needed — just like Miles calls for! 

Of course, as a control (and to cause more ballistic complications), we could then proceed to measure the BC at the same range with a Labradar.... 😄



Thanks for your comments, this is indeed complicated. 😊


Matthias

[K.O.]

well imo is possible small burr (and other defects) can make there be a small nose inwards skirt wobble that causes  only slight  effects accuracy wise...

sort of related to a bullet going to sleep after coming out of the barrel wagging its  tail... but not cause different mechanisms at play caused by the same-ish walls of air...and the cg of course forwards for the pellet...

among conditions the  round will face... badly under spun...to just a bit under spun...in the sweet spot...a bit over spun...radially over spun...

and the how each  slug/pellet design is affected by those conditions... also where it feel drag is moving forwards the faster the round is pushed...Some Schlieren Imaging if we could get enough quality would probably really help show the effect of different speeds on different pellets/slugs especially different nose/head shapes/meplat sizes...

wonder if High speed cam filming a pellet thru a cloud chamber would show anything...

here is some high speed Schlieren Imaging bottom vid in link... include a pellet from a pumper...surprise it has diamonds..

also it has a pic of .22 subsonic that the round appears to bow...could it be a result of yaw of repose + small amount
of wobble... or maybe some other type of artifact...

https://www.thefirearmblog.com/blog/2022/06/30/diy-slow-motion-schlieren-imaging-bullets/

also could do a dyi wind tunnel to do some of it in... seems it just might help refine our understanding of what is actual happening... if nothing else it sure would help illustrate some what we do know...

[K.O.]

radically not radially over spun... unnoticed spell ceck mis click..