Range Report Cortisol is and spin drift .308

[Knight6]

Hi, does anyone know at what sort of range a 175gr FGMM at 2650fps starts showing significant effects out of a 1:11.25 right twist barrel? Anything at or above 0.25 MOA. I realize it depends on direction N S E W. Also on altitude etc but is there a base for the calculations e.g sea level, 20 degrees C or similar? Or anywhere I can look up this data for varying range, atl' etc. not just looking for a calculator want to understand the theory and math.

Thanks.

 

[Trevor300wsm]

Are you referring to Coriolis? Or Cortisol......I doubt the latter would have any bearing on your bullet.

 

[TheGerman]

Ya my blood sugar is a bitch on my groupings.

 

[Deleted member 10043]

Hi, does anyone know at what sort of range a 175gr FGMM at 2650fps starts showing significant effects out of a 1:11.25 right twist barrel? Anything at or above 0.25 MOA. I realize it depends on direction N S E W. Also on altitude etc but is there a base for the calculations e.g sea level, 20 degrees C or similar? Or anywhere I can look up this data for varying range, atl' etc. not just looking for a calculator want to understand the theory and math.

Thanks.

The answer or answers are built into exterior ballistics theory as a whole so your question is way more than just open. The 175 FGMM is based on the 175 gr. Sierra MatchKing bullet. Based on the twist rate you are looking at a stability factor below 1.8 where you will start to see "signficant effects" in general. Formulas and sources are all over the internet for this particular bullet and stability factors. As for spin drift and CE, again there is a wealth of information on this as well. It takes about a semester of studying to get a handle on these sort of things. Bryan Litz's book is probably the best written book in beginner exterior ballistics. There are other books out that are good too. But Bryan has a lot of the basic math and explanations. Just wish it had a index, which sort of takes it out of the text book range. But it also forces you to find the answer. Nothing comes easy.

 

[jim_k]

A .25 MOA combined spin drift and coriolis will occur at fairly short range, but you will likely not notice it. That is, you will have a combined drift of more than 0.25 MOA at 400 yards (more than 1 inch total). We're talking northern hemishpere, but that is you where you are. The reason you won't notice the drift is that wind reading error, group size error, and sighting error at 400 yards will be more than 0.25 MOA, so the drift errors can't be demonstrated. You just have to plug in the calculations, and let 'er rip.

 

[Knight6]

Got to love auto correct and not double checking your text. Not sure about cortisol effects. Probably most effects are shooter based. I appreciate the input. I am pretty good with complicated math and physics and am interested in studying the subject. Thanks Culpeper and Jim K

 

[damoncali]

If you really mean "complicated" (as in you have the equivalent of a math or engineering degree), then check out Bob McCoy's book, "modern exterior ballistics". It is THE book on ballistics. Litz's book is by far the best of the bunch for people who prefer to skip the math.

 

[EXTREMEPREJUDICE]

I doubt the latter would have any bearing on your bullet.

LOL! Or the former, with a .308........................although opinions vary.........

 

[Deleted member 10043]

I'm just waiting for the apps to start having a gravity of the moon (tides) variable to add to the spin drift and CE. How can CE be accurate if the moon is causing a bulge on the ocean's surface?

 

[damoncali]

There's actually a section in McCoy's book that details how the curvature of the earth impacts the solutions (which assume that the earth is flat). It's a bigger number than I would have thought. I want to say about an inch of error at 1000 yards, but I'd have to look it up. The application there is obviously for very long range (artillery) stuff, but I thought it was interesting.