JSB .25 Heavy MK2 pellet BC at altitude

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qball

qball

Member
Apr 13, 2020
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CA, United States
Hey guys



I had the luxury of spending a few hours during my camping trip this weekend target shooting and plinking. The spot we camped at was around 6200 feet, I plugged in Strelok JSB Heavy MK2 and zeroed it in at 20 yards. But I kept on hitting way higher so I double check the altitude setting and everything seems to be correct. A little frustrated I decided to dial up the BC, started at .05 then .06, finally at .092 my POI and Strelok ballistic finally lined up for the most part. I was completely blown away! Is it possible that the heavy MK2 pellets could have that high of BC at 6200 feet or higher altitude?

To validate the BC increase and I have not completely screwed up settings in Strelok, I switched to .25 hades. It also shot higher than the Strelok but it lined up at .04 vs published BC of .026, however even at 1000 feet elevation I found the trajectory data lines up with my POI at .036. BUT .036 to .04 is not any where near the jump I saw with heavy MK2's at .048 to .092!

Anyone has similar experience? Any ideas on how or why?
 
You have to adjust for True barometric pressure altitude is not a substitute. Also you need temperature and humidity values as well.
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I tried to use Strelok/s feature that uses internal barometer and it didn't change the calculations at all. Can temperature and humidity change BC THAT much? How come it didn't affect the Hades nearly as much? Hades' BC changed from .036 to .04 which is well within reason but Heavy's BC almost doubled!
 
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A change in altitude has the greatest effect on the BC followed by temperature and humidity. They are all interrelated a little bit though. From 1000ft. elevation to 6200ft of elevation equates to 6.2” of pressure. I didn’t punch it into my ballistic calculator so I don’t know if .048 to .092 is accurate though. Seems like an awful lot!
Stoti
 
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As far as I understand the BC stuff.... —

making BC calculations based on change of POI is notoriously inaccurate.

And at short distances (20y) even more so. Better are say 60y, 10-shot group, no wind — find the mean POI (with flyers eliminated).

And make sure the atmospheric data is accurate.... — big difference between local atmospheric pressure (what Strelok needs) and pressure adjusted for sea level (what weather stations report). (There are ways to get from the latter to the former, or use the phone's barometric sensor.)



Glad you got to try out airgunning at such a high altitude, cool! When the covid-19 house arrest gets lifted in my neck of the woods desert I'd like to take a mountain trip.... Just two hours out of town at almost sea level the road hits 8500ft (2600m). I'll be sure to take a chrono along...! 😄



Matthias
 
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A change in altitude has the greatest effect on the BC followed by temperature and humidity. They are all interrelated a little bit though. From 1000ft. elevation to 6200ft of elevation equates to 6.2” of pressure. I didn’t punch it into my ballistic calculator so I don’t know if .048 to .092 is accurate though. Seems like an awful lot!
Stoti
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I was shocked to see the difference to say the least. This is my setup(I use mildot reticle): level 20 meter zero in target, 40 meter 1 inch metal targets, 65 meter 1 inch metal targets and paper targets for groups, all targets are laser range verified with Sig range finder. I also have a very large hill behind the targets and plenty of plinking targets at 80, 100 and up to 120 meters(pine cones, rocks, logs...) at different elevation/angle change. This gave me plenty of targets at different distances to verify ballistic calculations, Strelok pro and Sig BDX app and their calculations are basically identical. Hades were shot at 860 FPS and Heavies were shot at 830 FPS, Heavies seems to be a little more consistent with wind but both flew true and straight when I can see the pellet in flight, I'm really loving these pellets.



With both ammo I tried I zeroed in first and then try to hit the 1 inch metal flip targets, then the 1 inch metal targets and then paper targets at 65 meters. With the Hades I only made minor modification from .036 to .04 and just about everything lines up. At 100 meters the POI is about 1-2 mils or 1-2 centimeter lower which is easy to compensate for and well within range of margin of error. With the Heavies it took a while to get my 3 POI on my targets to line up with the calculator until I changed the BC to .092. With the new BC I was able to hit pine cones and small rocks on the hill between 100-120 meters at first shot or very very close. 



After thinking about it both Hades and the heavies have the new/redesigned shape. Published BC for Hades is .026 and I ended up at .04 and published BC for heavies is .048 and I ended up at .092. If I look at the delta between the published number and my actually number at elevation it seems to be equally proportionate and supports each others BC delta. The redesigned shape(less dramatic waist) seems to have really good aerodynamic and has higher BC than actual published number especially with thinner air.



Couple of my favorite shots were a 4 inch lizard at 44 yard at 7 degree incline, dead center head shot on the first shot and splitting a young pine cone(3-4 inch) in half 65 yards but 30 degree incline/way up in the trees. This goes to show how important ballistic validation is and how awesome it is once it is validated. I literally look for a target and range it then dial and shoot, even if I missed it's really close. I thoroughly enjoyed the plinking and ballistic validation session, definitely learned a lot!




 
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Not sure if this makes a difference but pilots use Density Altitude to measure the actual altitude conditions at a given time. Humidity, temperature, barometric pressure are used to calculate the air's density. It's not at all unlikely to see density altitudes of well over 10,000 feet above sea level at 6200' ASL on a hot summer's day. So the air is much thinner. Thinner air equals less friction / drop ?
 
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Not sure if this makes a difference but pilots use Density Altitude to measure the actual altitude conditions at a given time. Humidity, temperature, barometric pressure are used to calculate the air's density. It's not at all unlikely to see density altitudes of well over 10,000 feet above sea level at 6200' ASL on a hot summer's day. So the air is much thinner. Thinner air equals less friction / drop ?
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Very possible and good to know, guess my next purchase would be Kestrel wind meter with ballistics which as all of the sensors. Kestrel along with Sig is what matt dubbler uses and I can see why now. Need to save up my pennies again, at least I got the Sig range finder part!
 
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This is one of the reasons I brought the fx impact most of the BC calculations you find for pellets are derived from the pellet being shot out of an fx impact shooting it from a different gun would change that BC and in most cases involve a lower BC calculation so most pellets BC that you find on line or on the the tin are only valid when shooting out of a well tuned FX impact. So BC labels are pretty bogus but will get you close if you are shooting the same gun that was used to find the BC
 
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Sure, every barrel, every gun shoots a little different — so BC numbers will also have those variations.

However, the biggest difference for the BC for the same pellet/ slug seems to come from the velocity (muzzle to impact).

Bob Sterne did a very helpful graph for the domed JSB line in .22cal, cf. below. And Strelok Pro allows you to enter various velocity-dependent BCs.

Matthias



1592855271_3566805755ef10ae8010fc0.20926774.jpg

 
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Something is off, no way the 34 has that high a BC. 

My guess is you used the G7 BC instead of G1. I get 3.5 mils of drop at 100y with the 34gr with a G7 of .048 vs 4 mils for G1 of .048, both at 9000DA going 832 fps. 

This error would certainly screw the results.
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I left it on G1 because G7 BC doesn’t go down to pellet granularity. I used 2 separate ballistic calculators and the same calculators have been pretty spot on at 1200 feet. I also only changed BC in the same ammo profile, not a new profile. I’m at lost too.

1592968159_16831729205ef2c3df9924e0.31271450.png

as you can see Sig’s app automatically updated the altitude and temperature and validated with my buddy’s Garmin GPS unit. This is Hades’ data, probably could use a little more BC to align 100 meter shots better. 
1592968308_18937714685ef2c474ac19c8.02056611.png


Same environmental also in Strelok and lines up identical with Sig app calculation on both ammo so I assume calculation is correct and alteration to BC would align both app’s calculation with my POI at ranged targets up to 120 meter or 130 yards.
 
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Yes, definitely the GA profile as @ackuric states above. And even though the altitude does affect the BC, it doesn't affect it as much as the results you see above. In my experience, the BC rises slightly with altitude. I've calculated BC with the .25 Mk1 Heavy at around .048 to .050 at sea level, and at around .052 to .053 at 3500 ft. elevation. Using GA profile on a warm day around 75 to 80 degrees. And using the drop method measuring the drop at 100 yards with the zero at 50 yards, and using a chrony for measuring MV at about a foot from the first sensor. IMHO this is a much more "real world" way of doing BC than using the speed decrement.

Mike
 
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