Which BC calculator to use when using Strelok Pro?

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Chuck26287

Chuck26287

Member
Jul 29, 2018
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Indiana, United States
I've been using a LabRadar to collect velocity data at different distances. This provides good velocity data for the BC calculators on Pyramyd Air, AoA, etc. I've been using the Pyramyd Air BC calculator and determining my actual BC(s). I've felt the BCs I've been getting were on the low side, so I tried to use the Truing Function in Strelok Pro, but it displayed an error message, indicating I could not true the BC when a non-standard drag function is used. I'm using the Airgun Drag Function (GA) offered in Strelok Pro.

In thinking about this, I remembered that Strelok Pro indicated the GA drag law was from Chairgun. I went into the Chairgun app and calculated my BC(s), and they were much higher. Chairgun also indicated in the BC calculator it was using the GA drag law. So, my question is, do the other BC calculators on airgun sites not us the GA drag law, and therefore they are calculating much lower BC(s) using the same distance data?

I would assume if one is using the GA drag function in Strelok Pro, when determining your actual BC data, you should definitely use the BC calculator in Chairgun with the same GA drag function selected? To do otherwise seems to introduce error into Strelok Pro. Am I understanding this correctly?

Sample data (JSB .30 cal 44.75 gr shot to 50 yds):

Distance Velocity (fps) Pyramyd BC Chairgun BC

0 yds (Muzzle) 870.3

10 yds 841.8 0.0375 0.0434

20 yds 813.8 0.0372 0.0422

30 yds 787.8 0.0377 0.0417

40 yds 764.7 0.0387 0.0422

48 yds 747.2 0.0393 0.0426
 
Personally, I stick to G1 and use the truing functions. It's not as good as using a custom drag curve or GA BC by itself, but since Strelok lets you input multiple BCs at various velocities I use the multi-BC function and basically get a custom drag function that way. You could also just change the GA BC value manually until it's trued with your data, but I go the prior route since Strelok makes that easier.
 
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Chuck, those BCs that you got with the .30 cal. JSB 44.75 are pretty much spot on with what I get for that pellet with my .30 FX Bobcat and my .30 Vulcan2 Tactic. I usually use 0.042 and it comes out as close as can be expected. The GA function is the drag function that most closely approximates the diabolo pellet shape...

When you use the truing function on Strelok Pro you need to ensure that the zero is less than the yardage you are using for the truing. My normal distances for determining BC with Strelok truing are a 50 yard zero and the target or second measurement at 100 or 110 yards.

Mike
 
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Chuck, those BCs that you got with the .30 cal. JSB 44.75 are pretty much spot on with what I get for that pellet with my .30 FX Bobcat and my .30 Vulcan2 Tactic. I usually use 0.042 and it comes out as close as can be expected. The GA function is the drag function that most closely approximates the diabolo pellet shape...

When you use the truing function on Strelok Pro you need to ensure that the zero is less than the yardage you are using for the truing. My normal distances for determining BC with Strelok truing are a 50 yard zero and the target or second measurement at 100 or 110 yards.

Mike
Click to expand...

Hey Mike,

Yeah, I think the ones calculated with Chairgun using the GA drag function are good. Until today, I was using the lower BC values calculated from the generic BC calculators on all the airgun websites. Until I started suspecting my BCs were low, I had no idea there was a difference between the BC calculators. As soon as I ran the same data through Chairgun's calculator and came up with more realistic BCs, I knew there had to be a difference that I wasn't aware of.

It also bothers me that others seem to be able to use the Truing function with the GA drag function, whereas I can't. It makes me think I've got something else configured wrong. When I'm configured to use Multi-BC, and I try to true it up, I get the error message I quoted above. If I change it to Single BC mode, I don't get the error message, but I just get a BC of 0.000 showing after I hit calculate. I also zero at 50 yards, and entered 100 yards for my distance to true, then for testing purposes, I use both +/- 1.8 inches in the elevation window, and I get no correction calculated. I am not seeing what is not right, but it appears just using the GA drag function should not prevent this from working.
 
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You can’t use multi BC in truing. Multi is a waste of time anyway. When you’re zero’d at 50 yards and do the measurement at 100 yards it will be about 11 to 13 inches depending on your speed. How do you get 1.8 inches? You should get an EXACT zero at 50 yards. Then when you shoot at 100 yards, run the shots over the Chrony about 1 foot (for muzzle velocity) from the muzzle. Set the target at 100 yards. Shoot 5 (or more) shots. Aim at the top of the target. Get the average speed from the Chrony. Now go and measure the center of your group and how far down that is from your aim point. Should be around 12 inches like I said. That will go into the Strelok Truing BC function. 

19C85F5E-AF85-4ED9-A960-05517372BAA9.1604967134.jpeg

Don’t change your scope when you shoot at 100 yards. Leave it set where it’s zero’d at 50 yards. Then when you shoot at 100 yards you want to see how far the pellet drops at the speed you are shooting. Make sure you have the zero set in Strelok on 50 yards in the Scope section right at the top. .
 
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Okay... at least a few things going on that made things puke.

1) First and foremost... like CC said, you can't use Multi-BC when using the Truing Function. It was actually the use of the Multi-BC function that was "nonstandard" and causing the error message I was getting, not the use of the GA Drag Function.

2) There appears to be useful data tolerances in place that prevent a proper calculation if the data entered is too far from anything realistically useful. For instance, When zeroed at 50 yards, and truing the BC for 100 yards, when I entered either +/- 1.8 inches, it simply returned 0.000 for the new BC. However, if I entered 12.5 inches like CC mentioned, it DID properly calculate and return a new "trued" BC.

3) CC asked where I got 1.8 inches... well, NOT from a test shot to 100 yards. I was just trying to put in something close, but off enough it would cause a different BC to be calculated so I would know it was working. I realize 1.8 inches is not close to 12.5 inches, but here's where it becomes apparent this was, as it often is, pure user error. I thought the number I was to enter on the correction distance was how much the shot was off from where Strelok Pro said it should be. I wanted it to calculate a new BC for a shot that was 1.8 inches off what it was supposed to be, so I enter 1.8, where I should have been entering 14.3 inches (the expected 12.5 inches plus the extra error of the 1.8 inches).

So, thanks to everyone, particularly CenterCut, for helping this to be cleared up. I can now true the BC using the GA Drag Function.
 
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