Ballistic Coefficient for 357 Cal - JSB Match Diabolo and Predator Polymag

[frank320]

Searched and found little to no BC information on .357 cal JSB Diabolo and Predator Polymag pellets. So thought I would start collecting some data to share.

Chrono: Caldwell G2
Rig: W.A.R Flex 357, Cothran valve, Cobra regulated, unchoked barrel. Pellets lubed with FP-10.
Ballistic Calculator: G1 via http://www.jbmballistics.com/cgi-bin/jbmbcv-5.1.cgi
Temp 68, Humidity 50%, Altitude 500ft
*as a reference data point, I measured the BC for .25 cal JSB MK2 at 800fps at muzzle using my Armada under same conditions. BC came to 0.052, which matches what I see widely estimated for this pellet.

1) 357 JSB Diabolo with regulator set at 1900psi
Average muzzle velocity: 808fps(spread of 2fps)
Average velocity 25yds downrange: 735fps(spread of 4fps)
BC = 0.034

2) 357 Predator Polymag with regulator set at 1900psi
Average muzzle velocity: 809fps(spread of 2fps)
Average velocity 25yds downrange: 739fps(spread of 3fps)
BC = 0.036

3) 357 JSB Diabolo with regulator set at 2600psi
Average muzzle velocity: 882fps(spread of 2fps)
Average velocity 25yds downrange: 805fps(spread of 5fps)
BC = 0.038

4) 357 Predator Polymag with regulator set at 2600psi
Average muzzle velocity: 883fps(spread of 2fps)
Average velocity 25yds downrange: 811ps(spread of 4fps)
BC = 0.041

Interesting notes:
1) BC improved as velocity went up from 800fps to 880fps at the muzzle for both pellets
2) BC of 357 Predator Polymags were consistently better than JSB Match Diabolos
3) Spread of Predator Polymags were consistently tighter downrange than JSB Diabolos

It is surprising that the Polymags in .35 cal has a better BC and is more consistent than .35 JSB Match Diabolos. In the case of my .25 cal, the .25 cal JSB MK2 way outperform .25 cal Polymags in terms of BC and consistency by a wide margin.

If someone has better data or a better estimate of the BC for these pellets, please do share your data and experience.

Best,
Frank

[rsterne]

Interesting data.... I assume the Exacts you were using were the 81 gr., JSB made a 78 gr. earlier, but I think may have discontinued them.... Since the .357 pellet has a higher Sectional Density than the 34 gr. King Heavy .25 cal I would have expected the BC to be as high or higher.... On the other hand, proportion-wise it is much shorter, closer in shape to the .25 cal 25.4 gr. Kings, which have a BC of about 0.038.... I guess the shape in this case has a larger effect than the SD....

Bob

[frank320]

Hi Bob,
Both pellets were 81gr. The Polymags are longer in length than the JSB Diabolos. And in my case, the polymags shoots more accurately and more consistently too. The head sizes on the Polymags are also more consistent than the JSBs also -  well, the polymags do cost more too.

[rsterne]

I have never had a gun that shot the Polymags near as well as the Exacts.... and they always have a lower BC than the Exacts.... but then I haven't tried them in .357 either....

Bob

[frank320]

I have never had a gun that shot the Polymags near as well as the Exacts.... and they always have a lower BC than the Exacts.... but then I haven't tried them in .357 either....

Bob

Same here, all of my .177/.22/.25 rifles shot way better BCs with JSB Diabolos than Polymags - by a wide margin, i might add, so I was very surprised by the .35 results. Hence, i repeated the .35 JSB Diabolo BC test done above twice, and thrice for the .35 Polymags just to verify the results above.

Then I input the average of the 2 BCs for each specific pellet at 800fps and at 880 fps for JSB(0.036), and for Polymag(0.039) into Chairgun, lasered a 115yds target, dialed in the elevation for each pellet, and let them fly. Both pellets were spot on at 115yds with their average BC inputs i used from above - input into Chairgun was 0.036 for JSB and 0.039 for Polymag for muzzle velocity of 880fps shooting out to 115yds.

Weird huh? Additionally, from my limited testing, the .35 Polymags were more accurate and more consistent than the .35 JSB Diabolos all the way out to 150yds, the furthest i tried thus far. I had initially hoped that the .35 cal JSB would be my go to pellet, just like the rest of our air rifles, and they are cheaper too, but seems like the data shows otherwise in my case.

[ShakySarge]

The information I have for the .35 JSB is that the BC is 0.0540. That’s a significant difference from what you gathered. Is there an absolute equation to verify? I understand utilizing the speeds you are achieving to calculate, however, there should be a standard BC correct?

[rsterne]

BC is one of the things where equations don't really work.... In addition, depending on the drag model chosen, the BC does vary with velocity.... Just by comparison with other calibers of JSB Exact, where the BC is roughly 2/3rd of the SD, I would have expected a BC closer to the 0.054 value, however....

Bob

[ShakySarge]

Understood. Thanks for the input. I am getting anywhere between 0.054 and 0.053 depending on velocity.

[frank320]

I included the JBM ballistics link that I used to calculate the BC above in the original post. I also used the more conservative numbers to calculate the BC above, i.e I averaged down the lower velocity numbers at far end. I figured I rather input a lower BC number than over-estimate the BC in the ballistic app.

Having said that, one can take my BC calcs above and add 10% better. I have had success using those BC numbers out to 125 yards.

It’s good to have someone else run tests and compare BC numbers for these .35 pellets.

Also, what pellet speed are you capturing between the calculated interval? I think I was at 870fps to around 800 within the 26.5 yard measured interval.

[ShakySarge]

I believe I used 880 as my starting velocity with a drop to 800 at 50 yards. The 0.0540 was obtained from ChairGun so I couldn’t use it as a definitive answer. The 0.053 was obtained from a link that I will have to find again. Using JBM it translates to 0.020. I believe that is partly due to the type of projectile as well as many don’t have a diabolo or pellet option.

[ShakySarge]

Pyramyd air has calculators as well for it specifically for pellets. Theirs gave a slightly higher BC yet still closer to the .05 mark.

[Mildot_Spanish]

FX IMPACT M3 .35 CAL. CONFIGURATION:
REG1: 170 bar
REG2: 130 bar
VALVE: 4,5 LINES
MACRO HAMMER POSITION: 13
MICRO HAMMER POSITION: 4,3

SPEED / ENERGY RESULTS:
JSB EXACT 81 gr / G1 BC:0,038
271 m/s (889 fps)  /  192 J (142 fpe)

[rsterne]

I realize that this is a 4 year old thread.... but I didn't notice this at the time or I would have commented....

Pyramyd air has calculators as well for it specifically for pellets.

Their calculator uses a linear drag profile to calculate the BC.... While this works reasonably well in the Subsonic range, it is so wrong in the Transonic (Mach 0.8 - 1.2) that it is a  joke.... ALL projectiles have a huge increase in drag in that range, and using a constant Cd to calculate the BC of your pellet will give results FAR from reality.... The JBM Calculator is far better, as it uses the G1 drag profile.... Better yet is ChairGun or Strelok, using the GA profile....

Since this discussion is about .357 cal, I will add a table showing the results I obtained with my LabRadar....



My result, at BC (GA) of 0.038, is quite close to that obtained by the OP.... I did not have any Predator Polymags when I did that test....

Bob

[Mildot_Spanish]

ok thanks Bob
I didn't have predator polymags either. But I only use the 81 gr JSB exact anymore. To add a little more information regarding this pellet, I will show a little more results. In field tests I was able to measure the speed with a Caldwell G2 chronograph at 50 meters... I think this measurement establishes the ballistic coefficient very approximately at G1 0.038

MV =  271 m/s (889 fps)
V50 = 223 m/s (732 fps)

qBal software V100 = 186 m/s (610 fps)

All the best

[JungleShooter]

I'm glad there's people out there like you who do BC testing. 👍🏼


It never ceases to amaze (and disappoint) me how larger calibers often don't reduce wind drift by much....


It "costs so much more" to shoot a large caliber like a .357 pellet...: 
•It weighs 5 (five) times as much as my .22cal 15.89 grainer. 
•It requires a big blast of air from my smallish airtube.
•It makes way too much noise (one reason I use an airgun in the first place).
•And the pellet prices, well, let's not talk about such a depressing topic.

However, for all these costs I have little to show for in terms of wind drift resistance.

(Of course, I understand the other advantages of larger calibers, like making bigger holes in quarry with a lot more energy.)

(And I understand that the comparatively poor BC has to do with the sectional density.)



❓ I'm wondering if there is an "ideal" caliber (one for diabolo pellets, one for slugs) for achieving maximum BC — within the limits of airgunning?
Bob or Miles might even have a formula where you could plug in your specifics (I'd be interested in 400mm and 600mm barrels).

But to not derail this thread 😊 — let's discuss this separately, here:
https://www.gatewaytoairguns.org/GTA/index.php?topic=204619

Matthias

[jtipton91]

Ive had best luck with polymags