BC measuring (and optimal speed and pellet for long range shooting) /theorizing

@steve123, thanks for the vote of confidence.

Yeah this gun/barrel is an enigma. It's bad medicine on the prairie dogs, longest successful shots were back to back at 185 lasered yards. It was something like 12 mils of elevation and 3-5mils if windage. There were a couple running around right in that spot, took a couple shots to figure out the windage for the first one ,second one ran over to check out his dead buddy and I got him with one shot. Most of my little p dog sessions I'll get one or two somewhere past 140 yards with this gun. So, it'll hold accuracy together really good for really long range, just gotta be done with the 18.1s and they're not very good in the wind. Really having fun with it and the 18.1s. Cheap pellets and lots of shots/fill at the 35fpe, but want something not so susceptible to the wind. 

As for getting this resolved, I've got a couple unchoked LW blanks being delivered tomorrow for our buddy to machine one for this gun. If the new barrel doesnt like the 25.4 MRDs than I'm hoping it'll like an NSA offering (being unchoked). I stumbled upon the fact the other day that NSA slugs are now cheaper than the MRDs. 

The joy is in the journey.

Franklink,

I really enjoy your detailed reporting and clear writing.

I've tried to follow along your ballistic musings, and your idea of measuring the BC is excellent. I still have to get into that myself — waiting for a second chrono to arrive at my outpost....

I'm sorry that this barrel has not delivered what you wanted — with the tight choke and all.

I wonder if I have a similar problem with a LW — that hasn't done so well with two dozen different slugs I've tried — but all being .217 or .218, nothing smaller....

A thought on pellet selection:  Have you tried the H&N Baracudas? The Match version comes in three different head sizes, and they have a Power version (copper coated). 21.13 grain. BC similar to the JSB Heavy — or better due to its much higher weight.

About you measuring the BC: The 10 shots probably give you pretty reliable data — if the fluctuations between shots were not very large.

Would you confirm that you used ChairGun for PC to calculate the BC? And entered the atmospheric conditions at the test site? And selected the GA drag function? ➔ Because I would like to include your test in my Pellet BC Chart.

Here's another velocity-variable BC chart by Bob Sterne, with all the JSB domes in .22cal.

For longer ranges it helps a little to shoot the pellets a little faster than their optimal BC velocity — because during their flight they slow down and move to velocities with less than optimal BC.

Wind drift of 2 to 3 feet at 80 yards...? Wow, that's quite a storm — at least what ChairGun seems to tell me:

The JSB Heavy would drift 2 feet with a 30 miles per hour wind if directly from the side

— more wind if at a different angle.

3ft drift — 45 mph wind....

 



And then you had all of a sudden NO DRIFT?! Some crazy swirls and wind shear — sound like weather when even the birds take the bus. While you guys were shooting an airrifle competition...!! COOL!

Keep us in the loop on your quest for better long range precision, Franklink!

Matthias

Click to expand...

Thanks Matthias. 

I've tried slugs out of this barrel too, down to the 0.2165 size from NSA, the 17.5s to be specific. They did okay at < 50 yards but were really bad any further out. 

When I pushed various projectiles all the way through the barrel I was finding they measured 0.210-0.211. When I pushed them up to the choke and then back out the breech, they were in that 0.214-0.215 size. That suggests to me that even the 0.2165 slugs are too big for the choke on this particular barrel. 

Check out what a JSB 25.4 looks like after going through the choke.  

The marks made by the lands start pretty high up the skills of the skirt. Oh, and the amount of force required to push any projectile through the choke is just something else. Dang near has to be tapped through, vs pushed through. 

So, I think the tight choke keeps slugs from being an option with this barrel too. 

It's a good barrel and really likes the 18.1s. 

On your question of H&N, I tried these 



And they did just okay at 50 yards. From past experiences I have a really hard time making any H&N pellet a primary projectile for a gun. They've just seemed to be really inconsistent across various types and calibers, as well as from one tin to the next of the same pellet. 

As for BC collection, I just checked my notes and the SDs of the: 10 near for the lower power was 3.03, 10 near higher power 2.34, and each of the 10 far were a little over 6 (both high power and low power). So, fairly consistent. I use a single chronograph and move it. With a second chrono I'd suggest that you make sure they both read similarly or the BCs calculated like this won't mean much. I've seen quite a bit of variation from one particular chronograph to the next. 

No, I did not use chairgin for PC and the calculator I used does not have any specific drag profile. A drag profile might be built into the macro running the calculator behind the scenes, but the requested values are just distances and speeds. This is the one I used https://www.airgunsofarizona.com/Calcs.html but I've used the one on Pyramyd Air's website before too. 

And for the wind drift of 2-3 feet and what chairgun says the wind was doing that day.....it wasn't blowing as hard as chairgin says. With where I live, I have the opportunity to shoot at long range, a lot, and usually in the wind. I have settled into the stance that certain pellets from certain barrels at certain speeds can perform much better or much worse in the wind than ballistic calculators model. Along with that, I don't think that BC is the end-all, be-all to how well a projectile does on the wind. Yes, it's probably the best quantitative way we have to analyze how well a pellet/slug cuts through the wind, but it seems there is much more going on than what a BC would suggest. We really need something more qualitative than BC to describe how well a projectile slips through the air and resists wind deflection. And it doesn't seem like all the drag profiles are the answer either. I say that because certain gun/barrel/projectile combos just do much better than their BC says they should, when it comes to wind deflection. And vice-versa too, sometimes the wind really grabs hold of a certain type of pellet when the BC says it should be slicing through the air better than it does. 

Yes, I'll update how the new barrel performs. I've also got a power plenum coming for this gun. The extra couple inches of barrel and the plenum are sure making me look like one of those power hungry airgunners, something I never thought I'd be. I've fought it pretty hard but it seems that (consistent) long range accuracy will require a bump in power and a heavier more streamlined projectile. 

Matthias, I got nothing on Miles and Bob. I've read much of their stuff and they're leaps and bounds ahead of the rest of us. 

Yes, atmospheric conditions will influence the BC. I never tried to say anything contrary. Elevation is 5600 feet here. Humidity was probably around 10% (normal for us) and it was in the mid 80s (farenheit). BCs that I usually use when playing with Strelok or Chairgun are much better than what I typically see reported, and I fell that it's my thin (-er than sea level) air.

From your comments I am getting the impression that you're putting A LOT more faith in ballistic calculators than they deserve. A few comments on why I say that......

As I've mentioned on the forum before, I'm a pharmacist and I work in a hospital (think IVs). With the more complicated to dose IV meds, the hospitalists (inpatient physicians) almost always just order "pharmacy to dose x." That is the hospital's protocol for them turning it over to us. These drugs typically have complicated pharmacokinetics (what the drug does in the body). Many of them can cause serious toxicities if drug levels in the body rise above certain levels, most of them also don't have the required effect if drug levels aren't high enough, or aren't high enough for a certain amount of time (concentration-dependent vs time-dependent). So, keeping drug levels where they should be, in a patient with a constantly changing level of hydration and varying degrees of kidney function is fun. We often use dosing calculators that are very much like the ballistic programs we use in shooting. The dosing calculators allow for inputting all the parameters (age, weight, sex, kidney function, desired drug level, etc) just like we would do for Strelok for example (pellet weight, speed, elevation, temp, wind speed and direction, scope height above bore, etc). Often the dosing calculators get us close, just like Strelok does, but never exactly correct. Vancomycin for example, more serious infections require trough levels of 15-20. The dosing calculator will say that a certain mg dose, given a certain amount of hrs apart will give a trough of, say 16.7, with all the patient's parameters entered. After the patient has reached steady state (different for every drug but vanco is prior to the 4th dose) we draw an ACTUAL blood level (trough) and it is rarely 16.7. It will sometimes be supra or subtherapeutic and we adjust the dose and go again. 

When I work up a gun for a field target match, which requires knowing EXACT impact points from 10-55 yards, I have yet to have a gun/scope/pellets impact points match up exactly to what Strelok or Chairgun predicts. I've had various guns be close, but not close enough that I trust them for competition. I shoot all the guns that I will compete with at all of the distances, from the position I use for shooting, and record where the pellets are striking. I'll repeat that as many times as I can in varying conditions and usually what makes it to my dope card is an amalgamation of all the results from the shooting sessions. 

My two examples above are just trying to make the point that ACTUAL data trumps PREDICTED data every. single. time. 

Ballistic calculators are fun to play with, but truly shooting a specific gun will give data instead of predictions. 

So, the BCs I worked up the other day are pretty good ideas of how well the 18.13s slipped through the air that day, at those two specific speeds. They are actual BCs of this gun, calculated from actual data. I was using 0.040 in Strelok to get things to match up kinda closely, prior to doing any of the BC collection that started all of this discussion. Which lead me to this.....(just did this now as an experiment). 

Plugged all the numbers into an old version of Chairgun for PC.



A difference of 0.0016, using the GA, and STILL an inconsequential difference in BC from one speed to the next with the 18.1s from this gun. The difference using AOA's calculator was 0.0018. As I previously mentioned, I'm betting there's a GA drag profile in the macro/behind the scenes calculations for the AOA website tool. Th difference of 0.0002 between the calculators is most likely due to how it was formulated to do the rounding. 

Regardless of the calculator I use, the answer the question I wanted answered (will lowering the speed help the wind drift?) is pretty much the same. No, it won't. Now, had I seen a BC go from 0.0386 with the 35fpe to 0.045 with the 30fpe, you can bet your bottom dollar I'd be out there shooting it at distance to see how much it really drifts at 870fps vs 940fps. But, with this minimal (and very similar between two calculators) difference, nope, not worth my time. 

You wanna really have some fun? check out these numbers with G1 instead of GA.



G1 is a flat based spitzer, a far cry from a skinny waisted diabolo like the 18.13gr JSB, yet the predicted BCs are nearly the same. THIS is why I don't give much credence to putting all my eggs in the ballistic modeling program basket. They're a good start, but that's it-gotta shoot real pellets (or slugs) at real distances in real conditions to get real results.

Agree 100%

Strelok for shooting critters at long ranges (laser em and then see what the hold over/click value is) and actual impact points for real precise stuff like field target. 

Barring whatever the windage does to you, Strelok is pdog/ground squirrel accurate for sure, as long as you've got good numbers entered (crap in =crap out).