spin drift 308 and .50 bmg caculations?

[*greenhorn*]

i have been looking all over the internet for spin drift calculations for the 308 and 50 bmg, i thought about buying the iphone program jbm ballistic but from what i see it auto calculates but doesn't display the adjustment, this may be wrong i have not seen the program in person. does any one have a relabel calculation, web page, anything? im mainly trying to understand spin drift and its calculation i am going to be shooting 308 out to 1000 yards and am looking at the Vulcan armament .50 bmg as well.

respectfully

p.s. i posted in semi-auto as well that was a mistake

 

[Jhnmdahl]

Re: spin drift 308 and .50 bmg caculations?

Instead of starting a new argument, you could just read the last one:

http://www.snipershide.com/forum/ubbthreads.php?ubb=showflat&Number=674586&page=1

 

[*greenhorn*]

Re: spin drift 308 and .50 bmg caculations?

Not trying to start augment trying to find out the calculation or equation fOr it or a location to look have been to library but all those ballistic books pulled from "shelves" from what the liberian said

I like to know the math I understand it's complicated but the computer programs just telling you what it is I'd like to understand what's going on not what my phone tells me

Respectfully

 

[mouse07410]

Re: spin drift 308 and .50 bmg caculations?

If you go to JBM Online and input your parameters with zero wind - you'll get your spin drift chart.

The thread referred in the other post has some good info that's hard to extract because of the sheer volume of fluff surrounding it. My take-outs from that thread:
<ul style="list-style-type: disc">[*]At shorter ranges (e.g., 600 yards) spin drift is small - say around 1.5" to 2" for 308 SMK 175gr. At 1000 yards it can become 11"-12" for the same projectile.[*]Operator error usually is much more gross than spin drift. <span style="font-style: italic">So Lowlight has a good point: improve and perfect your fundamentals, as without them the finer details like spin drift won't help you get even close to the target.</span>[*]Errors in wind call usually overshadow spin drift. <span style="font-style: italic">Yeah, so? I should ignore one predictable element (accounting for which costs me nothing) because there likely are larger less-predictable ones? BS.</span>[*]My calculators adjust for spin drift for free, so it's included in my calculations by default (checked one box in the configuration). If I err - I err.[/list]

Book-wise there's no better book that the one by Robert McCoy. But unless you're a mathematician you'll have no joy reading it. Bryan Litz "Applied Ballistics" is a good book (there's Group Buy running now on the 2nd Edidion

). Bryan says that

Drift = 1.25 * (Sg + 1.2) * ToF^1.83;

where Sg is gyroscopic stability by Miller twist rule, ToF is Time of Flight (raised to power 1.83).

 

[*greenhorn*]

Re: spin drift 308 and .50 bmg caculations?

thank you

 

[Sendero_Man]

Re: spin drift 308 and .50 bmg caculations?

best advice I'd give is go out on zero value wind day and shoot at 600 and 1000 and see what your rifles do.

If you have right twist barrels, hits should be right. IF you shoot in a left to right full value wind, hits will be low and right. If you shoot in a right to left wind, hits will be high and left. Just the results I have seen in my own experiences.

I personally have my rifles 1/2 MOA left at 200 yard zero and it pretty well takes the spin drift outta the equation and still is a dead critter..

Works in my shooting, not a way to set up for everyone, but works great for what I do.

 

[seanh]

Re: spin drift 308 and .50 bmg caculations?

Though I've been recently looking at various formulas for external ballistics, I'm remembering when we didn't care nor did we need it. We simply shot...a lot. We got to know our guns and our rounds. There was no mil drops or wind clicks...we simply held what we need. Most of the time we hit what we wanted.

here is a quick tutorial
http://en.wikipedia.org/wiki/External_ballistics

here is something from e how. I can't find the rest of my detailed data but this will do.

Determine the gyro stability factor (SG) of your bullets and firearm. Ask for this information when you purchase bullets. If the vendor does not know, there is a calculator that can help determine the SG. The SG should ideally be equal to. or higher, than 1.4.

Determine the time of flight at the distance you are shooting

Calculate drift with the following formula: drift = 1.25 x (SG + 1.2) x TOF ^ 1.83. Start by raising your time of flight to the power of 1.83. In the example, 1.9 raised to the power of 1.83 is 3.237. If the SG of our bullet and gun is 1.4, then the equation becomes: drift = 1.25 x (1.4 +1.2) x 3.237 for a result of 10.52. This means in this example, the bullet will drift 10.52 inches to the left or right depending on which twist the gun barrel has.

 

[Gunn-Fighter]

Re: spin drift 308 and .50 bmg caculations?

Awesome thread, I have really learned alot from you guys. keep this on going

 

[mouse07410]

Re: spin drift 308 and .50 bmg caculations?

<div class="ubbcode-block"><div class="ubbcode-header">Quote:</div><div class="ubbcode-body">Though I've been recently looking at various formulas for external ballistics, I'm remembering when we didn't care nor did we need it. We simply shot...a lot. We got to know our guns and our rounds. There was no mil drops or wind clicks...we simply held what we need. Most of the time we hit what we wanted.</div></div>
And today it's multiple guns/multiple calibers and multiple rounds/loads for each - with different purposes and different ballistics. With no fixed area of operation (which means - can't really learn the predominant winds/seasons, etc). With the need to hit on the first shot, preferably better than most of the time. Probably at greater distances - though I can't know that. Preferably before tons of brass and years of time are spent.

So the amount of time allowed to learn your gun and your loads is shorter, the amount of rounds you can spend figuring this out is less, the demand for results is higher. Luckily, Science & Technology are here to pick the slack and help dealing with those.

Times change, you know.

 

[*greenhorn*]

Re: spin drift 308 and .50 bmg caculations?


i just wanted to know the equations im trying to get a deeper understanding, im learning from scratch and don't have anyone but you guys to ask, their so much crap on the net, or you input spin drift in yahoo and you see Michael Marelli taking his moms dress off??????? the local library don't have nothing because of the world we live in today, if you ask about a gun book they think you going to shoot someone

in the military they said "Macleod" shoot that and i did but i never understood why i could, "Kentucky windage" was my friend and i can adjust on the fly pretty fast. just trying to get a better understanding

I really pretreated all your comments, if this is such a common topic, maybe we should stickie this or have some one create a sticky of the spindrift calculations. but im sure most people plug it in to their ipad and "BOOM" 9 moa left and 38 moa up and "fire". one day you might need to figure on fly and wont have the ipad.

 

[seanh]

Re: spin drift 308 and .50 bmg caculations?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: mouse07410</div><div class="ubbcode-body"><div class="ubbcode-block"><div class="ubbcode-header">Quote:</div><div class="ubbcode-body">Though I've been recently looking at various formulas for external ballistics, I'm remembering when we didn't care nor did we need it. We simply shot...a lot. We got to know our guns and our rounds. There was no mil drops or wind clicks...we simply held what we need. Most of the time we hit what we wanted.</div></div>
And today it's multiple guns/multiple calibers and multiple rounds/loads for each - with different purposes and different ballistics. With no fixed area of operation (which means - can't really learn the predominant winds/seasons, etc). With the need to hit on the first shot, preferably better than most of the time. Probably at greater distances - though I can't know that. Preferably before tons of brass and years of time are spent.

So the amount of time allowed to learn your gun and your loads is shorter, the amount of rounds you can spend figuring this out is less, the demand for results is higher. Luckily, Science & Technology are here to pick the slack and help dealing with those.

Times change, you know.

</div></div>



You made the point. "I need it now" without developing skills. What happens when the battery goes out on the toy? Justify the purchase all you want, nothing wrong with having all the toys you feel you need. Me, I'd rather spend the money on ammuntion and developing my skils as a shooter.

come to think of it your not doing anything differnt than I do/did. difference is I'd just remember it and almost naturally compensate for drop, windage etc. I'd use the charts or formulas to get an idea of where the bullet was going, then I shot enough to prove it...I like to shoot.

crap, sorry for taking over the thread...

 

[mouse07410]

Re: spin drift 308 and .50 bmg caculations?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: seanh</div><div class="ubbcode-body">You made the point. "I need it now" without developing skills.</div></div>
Partially correct - as should be expected. Developing what skills, and expending how much time and money on them? For some shooting isn't the end-goal.

<div class="ubbcode-block"><div class="ubbcode-header">Quote:</div><div class="ubbcode-body">What happens when the battery goes out on the toy?</div></div>
What happens when you run out of ammo? <span style="font-style: italic">Use the battery-powered radio to call for a pickup.

</span>

Some people bring MilDot Master, FDAC slide-rule, printed trajectory or come-ups charts etc. as a backup in case their ballisticomp dies (for whatever reason). Makes sense.


<div class="ubbcode-block"><div class="ubbcode-header">Quote:</div><div class="ubbcode-body">Justify the purchase all you want, nothing wrong with having all the toys you feel you need. Me, I'd rather spend the money on ammuntion and developing my skils as a shooter.</div></div>
Nothing wrong with that either. Especially if you have enough space (distance) to shoot, enough money for the ammo (308 isn't as cheap as it used to be, and 338LM just burns the banknotes), and enough time to dedicate to the sport - good for you.

From what I see, most people now are more dependent on technology than before, but on the other hand this technology opened possibilities that just weren't there.

For example, I'm sure you're figuring out the distance "by eye". Great, useful skill. I also try to guess the distance, but whenever I can I lase the target.


<div class="ubbcode-block"><div class="ubbcode-header">Quote:</div><div class="ubbcode-body">come to think of it your not doing anything differnt than I do/did. difference is I'd just remember it and almost naturally compensate for drop, windage etc.</div></div>
If you can naturally remember drop, windage and such for a few loads in say two or three calibers (308, 300WM and 338LM come to mind) for all the ranges within 1000m for 308, 1500m for 300WM and 2Km for 338LM, and for all the weather conditions - not just good, great for you! I know I can't practice enough to have that memorized. I'll be happy if I just get fundamentals to the level Lowlight calls "professional".

<div class="ubbcode-block"><div class="ubbcode-header">Quote:</div><div class="ubbcode-body">I'd use the charts or formulas to get an idea of where the bullet was going, then I shot enough to prove it...I like to shoot. </div></div>
So do I - but I don't have enough time, money and space to shoot enough. So I rely on the math more, and work on the fundamentals so that my limited skills won't offset the math too badly.

 

[*greenhorn*]

Re: spin drift 308 and .50 bmg caculations?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: mouse07410</div><div class="ubbcode-body"><div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: seanh</div><div class="ubbcode-body">You made the point. "I need it now" without developing skills.</div></div>
Partially correct - as should be expected. Developing what skills, and expending how much time and money on them? For some shooting isn't the end-goal.

<div class="ubbcode-block"><div class="ubbcode-header">Quote:</div><div class="ubbcode-body">What happens when the battery goes out on the toy?</div></div>
What happens when you run out of ammo? <span style="font-style: italic">Use the battery-powered radio to call for a pickup.

</span>


well said

Some people bring MilDot Master, FDAC slide-rule, printed trajectory or come-ups charts etc. as a backup in case their ballisticomp dies (for whatever reason). Makes sense.


<div class="ubbcode-block"><div class="ubbcode-header">Quote:</div><div class="ubbcode-body">Justify the purchase all you want, nothing wrong with having all the toys you feel you need. Me, I'd rather spend the money on ammuntion and developing my skils as a shooter.</div></div>
Nothing wrong with that either. Especially if you have enough space (distance) to shoot, enough money for the ammo (308 isn't as cheap as it used to be, and 338LM just burns the banknotes), and enough time to dedicate to the sport - good for you.

From what I see, most people now are more dependent on technology than before, but on the other hand this technology opened possibilities that just weren't there.

For example, I'm sure you're figuring out the distance "by eye". Great, useful skill. I also try to guess the distance, but whenever I can I lase the target.


<div class="ubbcode-block"><div class="ubbcode-header">Quote:</div><div class="ubbcode-body">come to think of it your not doing anything differnt than I do/did. difference is I'd just remember it and almost naturally compensate for drop, windage etc.</div></div>
If you can naturally remember drop, windage and such for a few loads in say two or three calibers (308, 300WM and 338LM come to mind) for all the ranges within 1000m for 308, 1500m for 300WM and 2Km for 338LM, and for all the weather conditions - not just good, great for you! I know I can't practice enough to have that memorized. I'll be happy if I just get fundamentals to the level Lowlight calls "professional".

<div class="ubbcode-block"><div class="ubbcode-header">Quote:</div><div class="ubbcode-body">I'd use the charts or formulas to get an idea of where the bullet was going, then I shot enough to prove it...I like to shoot. </div></div>
So do I - but I don't have enough time, money and space to shoot enough. So I rely on the math more, and work on the fundamentals so that my limited skills won't offset the math too badly.
</div></div>