Surprising BCs

Boy that is a piece of evidence that we needed last week. Do you recall when Strelock plugged in .046 on the Cricket... Neither of us would buy it. Never stop learning. Good stuff Mike.

Thanks, Mike, that's the kind of data that's helpful to many of us!

Thanks for the good info.

Yep. I did the initial .22 calc and used 60 vice 59 yards, but corrected it above. .0434 not .046. But still much better than .035. The hardest part was not hitting the chronograph in that wind. This wasn’t a breeze, it was full out blow your tripod over windy. My gun cases were blown off the picnic tables. If the EBR 100 yard final was shot in this wind the winning score would have been 675 or less.

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CCut.

0.043 is pretty much what you should expect at 3600 ft altitude for 1 to 60 yd depending on temp and RH. It is very much in the zone I get at 3680 ft alt at the farm again depending on temp and actual air pressure. Yesterday I did a bc with those pellets for 0.042 over 0 to 40 yd but would have shown a little higher over 60 yd ... next pg ...

However at sea level (denser air) that bc will come down to the 0.034- to 0.036+ zone again depending on temp and rH ; and all of course depending on what help or hinderence the pellet in transit gets from the prevailing wind as in tending towards 6:00 or 12:00 o'clock. A barometer is a good investment for any who like the finer print of ballistics as day to day pressure variation does make a difference even at the same altitude. Any good bc calculator should have pressure input facility.

Best regards, Harry.

Thanks Harry. I usually get about .038 at sea level in warm weather, say 75 to 85 F.

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Yes that sounds right to me. At that temp hence the "+" sign above.

My results come from the old Excalibre (yesterday) and the BSA Hornet. ... H.

Just did a quick 7 shot string over the Labradar from the old Excalibre.

Temp 88 F, RH 48%, PRESSURE 26.52, same 18.1 gr .22 JSB. Altitude is 2680ft.

Average bc = 0.0435 range 2 - range 1 = 40 yd. Edit: mv 850 fps.

The 7 shot group at 50 yd poi is 0.35" ctc.

Best regards, Harry.

One of my other surprises was how low the .30 BC was for both pellets, especially the 50.1 grain. Perhaps the ST barrel has something to do with it?

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This is possible, but in my opinion somewhat unlikely. 

First, engraved rifling tends to reduce ballistic coefficient, which is undesirable. Increasing ballistic coefficient is one of the (alleged, but never demonstrated to my knowledge in airgunning) benefits of a polygonal bore such as the ST barrel. 

Second, my expectation is that this effect would be relatively subtle either way. If it were more significant, I'd expect everyone to be using polygonal barrels...... 



If I were to guess, and I'm just guessing, I'd say the issue is that too much weight (pun intended) has been placed on sectional density when it comes to calibers.

The frontal surface area of a .30 caliber pellet is 46.60 square mm. For a 50.15 grain pellet that'd make for a sectional density of 1.0997 grains per square millimeter. 

The frontal surface area of a .22 caliber pellet is 23.76 square mm. For a 18.13 grain pellet that'd make for a sectional density of .7631 grains per square millimeter. 

That 25% is significant, but nothing increased aerodynamic efficiency can't overcome, and that efficiency will ultimately probably come from longer pellets. I'm guessing this is why there was initially a run up the calibers to .30 for competition, and then a run back down them to longer heaver .22 and .25 caliber pellets. 



Just my 2c. 

So there are a lot of different factors running around in strelok which, in my opinion, are poorly explained and generally not well understood. I'll give you two examples. 

Under the cartridge, there is something called the "temperature sensitivity factor." In bullets, powder burns better as it gets hotter, and more poorly as it gets colder. So this setting, to the best of my knowledge, is for your specific powder's temperature compensation. Most people I suspect leave it at the default, which IIRC is 1.4%. But should we really have that in regulated airguns? I shoot in some temperature extremes, and it stands to reason that there would be some factor as colder denser air will have greater viscosity, so there should be some temperature compensation. But what? I haven't the foggiest. It isn't an easy thing to do really good science on because you have a significant temporal separation across data collection points...... unless you happen to live next to a giant walk-in freezer that someone will let you shoot airguns inside. I have mine set to .1%, mainly because I just don't know what better to use. 



Another is under your weather inputs. You have altitude, temperature, and pressure. The pressure though is almost always around 30inHg, no matter where you go. How is this possible? Because for reasons that I'm sure make sense to meteorologists, units of pressure are altitude corrected. Don't believe me? Take two examples. 
Leadville, CO is at 10,000 feet. 
https://forecast.weather.gov/MapClick.php?lat=39.2508&lon=-106.2926

Meanwhile New Orleans, LA is about 8 to 10 feet below sea level. 
https://forecast.weather.gov/MapClick.php?lat=29.9537&lon=-90.0777

The documentation for Strelok is little help in this regard (http://www.borisov.mobi/strelokplus/manual.html) but playing around with the data a bit, I do believe the inputs for pressure are also meant to be compensated thus match what is on the forecast. I'm perfectly happy to be wrong about this though, because it'd mean I had a definitive answer one way or the other. 





At the end of the day, a ballistic calculator just takes a series of estimated inputs and spits out a mathematical guess. Its foibles and inaccuracies annoy me, but it is still remarkably good and I try to appreciate it for the tool it is. For what it is worth, I have no idea where I pulled the number from, but I've been running .043 as the BC for my JSB 18.13s for ages now. Seems pretty close to right for a dude living/shooting between 0 and 5,000 feet of altitude anyway. *shrug* 

Thanks for the input Harry. Just to clarify does this mean Strelok uses adjusted or actual atmospheric pressure? And is there better documentation for the app somewhere I'm not aware of which clarifies this and about a million other little questions I have on the software? 

Thanks for the input Harry. Just to clarify does this mean Strelok uses adjusted or actual atmospheric pressure? And is there better documentation for the app somewhere I'm not aware of which clarifies this and about a million other little questions I have on the software?

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I don't normally use Strelok but do have the app which I just looked over.

On the input data page there is a "Weather" heading which allows input for altitude, temperature and pressure in hPa. That would be actual ambient P.

If you don't know the T and P you can choose to allow Strelok to " Set standard pressure and temperature for this altitude" ie., the altitude you have determined. Now the standard figures will be approximations of the true figures, the latter true figures being those which are actually prevailing according to the real temperature and pressure.

That is where the final results given may vary from true results by a little or relatively a lot. If we think of "standard" as a kind of average around which actual weather and so ambient conditions, seasons included, vary, then it becomes obvious that to get results that are specific in quality and quantity we should try to take account of true quality data.

Hope this is somewhat helpful. Kind regards, Harry.

Excellent info here guys, thanks! Would love to know what the BC on the .22 RD Monsters is at 900fps and 940 or so fps. ...?