ballistics question

[AKSavage]

Let's say I use my angle compensating range finder to range something at a steep angle either above or below me and the rangefinder returns an angle compensated reading of 500 yards. Lets say the actual (line of sight) distance is 600 yards. (Simple geometry says that the angle I'm shooting downward is about 34 degrees, but that may or may not be relevant here. Let's say there's a 90 degree crosswind of 10 mph.

I know, from a ballistics standpoint, that I hold vertically for the *horizontal* distance, which the range finder has already calculated to be 500 yards.

My question is what do I hold for windage? The bullet still travels to a target 600 yards away, so do I use the windage hold for a 600 yard shot?

I have the Applied Ballistics app on my phone but it requires the "look angle" to the target, which my rangefinder (Sig Kilo) does not supply. I can calculate it from the line-of-sight distance vs the compensated (horizontal distance) but then I need to switch back and forth, then calculate, before shooting. I usually just print a range card for the altitude and temp I will be hunting in.

 

[bill123]

According to kilo 2000 manual in LOS mode the display toggles between LOS distance and angle.

 

[AKSavage]

Wow. Good to know. I have always just kept it set on the ballistic range mode. That will help of I have my phone with me, so I'll be able to use the AB app to get hold-over and windage.

However, up here in Alaska, since there's about zero cell service in most places I hunt, and especially if I'm going in light, I leave my phone at home. So I guess I'm still curious: hold vertically for a 500 yard shot and windage for a 600 yard shot?

 

[bill123]

So I guess I'm still curious: hold vertically for a 500 yard shot and windage for a 600 yard shot?

I would wait for confirmation from others, but as I understand it, that is correct.

 

[Canadian Bushman]

..... So I guess I'm still curious: hold vertically for a 500 yard shot and windage for a 600 yard shot?

Yes

 

[Mikecr]

I would be careful here.
Wind is not based on distance, but time of flight(TOF) in a wind vector.
Bullet vertical path is not based on distance, but on your aimed angle, velocity/BC/TOF.

The most accurate slope shooting applies ANGLE bias to PATH(a path adjustment). That is, it should be more accurate to enter your shooting angle(either from ranger, or ACI), than slope distance, into ballistic calcs.
A good ballistic program would adjust path, based on slope angle, and apply resultant TOF in wind calculation.

 

[THEIS]

I would wait for confirmation from others, but as I understand it, that is correct.

Hello,

Correct...

The horizontal range elevation must be adjusted for MET and ENV conditions before the slope corrections are applied. Air density corrections are effectively adjusting for different time of flight to the target, and that altered time of flight applies on slope shots as well as level ones. The correct calculation sequence is:

elevation (R, θ, MET) = (elevation(R,0) MET adjustments) * Cos θ

THEIS

 

[AKSavage]

The idea of wind drift being based on time of flight (TOF) makes sense, as does the idea that TOF is not purely a factor of line-of-sight distance, and that to be *precise*, corrections will need to be calculated in.

A very interesting discussion! Thanks to those with amazing knowledge who responded!

My take-away from this is: I'm thinking that as an ESTIMATE of the correct windage, I could use windage listed for the line-of-sight distance on my range card. In other words I would dial the elevation listed for a 500 yard shot and the windage listed for a 600 yard shot. I'll have to run some numbers to see at what point that estimate becomes too inaccurate. I'm thinking that maybe for my example it might still be close enough, but by the time one gets to 1000 yards and beyond the error may become too large.

This is where a rangefinder with built-in ballistics software becomes appealing. Instead of carrying a rangefinder AND a smart-phone with a ballistics app, and entering the output from the rangefinder into the ballistics app, the rangefinder gathers all the inputs for you (distance, pressure, temp, and in some cases even wind speed/angle) and immediately outputs the elevation and windage to dial/hold. Something tells me within a few years high-end rifle scopes will do that ALL for us and we'll be able to leave the rangefinder at home too. Sniper's Hide has a thread on just such a scope at the SHOT show this year, although I don't think it has a laser rangefinder built in yet.

 

[Mikecr]

Just ran some numbers with a .628 G1 BC at 3Kfps, 100yd zero, full 10mph wind vector.

- MOA corrections for 600yds flat shooting: Elev 10.83, Wnd 3.13
- MOA corrections for 600yds 34deg slope: Elev 8.45, Wnd 3.13
- MOA corrections for 500yds flat shooting: Elev 8.09, Wnd 2.53

If you used 600yds for wind you'd at least be right there.
If you simply used the 'rifleman's rule' slant range for both (500yds flat), you be off by .36moa Elevation, and .60moa Wind,, both significant errors at 600yds.

 

[RockyMtnMT]

The time of flight for distance does not change with the angle of the shot. The amount that gravity effects bullet flight decreases for up or down shots at the same rate for the angle of the shot. Wind must be calced for the lazered distance then compensate for angle for the elevation. My system is old school now with all the great new stuff but takes care of this scenario. I have my ballistics program on an ipod and use range finder and weather meter to give it the dope. once dope is in I set the ipod on the top turret of the scope while the rifle is aimed and the los changes the drop needed.

Great thread.

Steve

 

[THEIS]

Sniper's Hide has a thread on just such a scope at the SHOT show this year, although I don't think it has a laser rangefinder built in yet.

Hello,

O you must be talking about the new scope from Chris Thomas (Premier Reticles Family) that he has been working on while at Gunwerks Product Development

Glad to see the release of this scopes information and even more glad to see Chris T back in the swing of things.

THEIS

 

[AKSavage]

Just ran some numbers with a .628 G1 BC at 3Kfps, 100yd zero, full 10mph wind vector.

- MOA corrections for 600yds flat shooting: Elev 10.83, Wnd 3.13
- MOA corrections for 600yds 34deg slope: Elev 8.45, Wnd 3.13
- MOA corrections for 500yds flat shooting: Elev 8.09, Wnd 2.53

If you used 600yds for wind you'd at least be right there.
If you simply used the 'rifleman's rule' slant range for both (500yds flat), you be off by .36moa Elevation, and .60moa Wind,, both significant errors at 600yds.

You beat me too it. This is exactly what I had in mind. As I read these numbers, (and correct me if I'm wrong) my windage would be dead on if I hold windage for a 600 yard flat shot. I wish I could say my hunting rifles shoot less than .36 MOA (less than 2.5" diameter group at 600 yards), but most are good to .5 MOA at best.

Keeping in mind that when we say a rifle shoots .5 MOA that means +/- .25 MOA, let's add some more wiggle room and say my rifle (and I) only shoot 1 MOA groups (+/- about 3") at 600 yards. That means if I dial the "range card elevation" for a 500 yard flat shot (8.09 MOA) and windage for a 600 yard shot flat shot (3.13 MOA) my windage error would be at most .5 MOA = 3" (all rifle error) and my elevation error would be *at most* .86 MOA = 5.25" (.5 rifle + .36 calculation error).

That is to say: the calculation error is a LOW miss by .36 MOA, so the *worst case scenario* is a hit about 5" below point of aim. Best case scenario, the rifle shoots .5 MOA high and the calculation error puts me .36 MOA low, and POI would be .14 MOA (3/4") ABOVE point of aim. The 5" worst case low miss would be pushing the limits of what I can shoot using just a printed range card. Good to know your limits.

 

[Mikecr]

I believe you're on the right track.
Just wanted you to slow enough to consider actual potentials.
34deg is a pretty steep slope, maybe worst case. Also keep in mind that my example numbers are not yours(different MV, BC, etc.)

 

[AKSavage]

I believe you're on the right track.
Just wanted you to slow enough to consider actual potentials.
34deg is a pretty steep slope, maybe worst case. Also keep in mind that my example numbers are not yours(different MV, BC, etc.)

Sure thing. I appreciate your input and expertise. This has been a fun thought experiment, and I hope others on the forum benefit from it too. I still intend to run my own numbers, although mine are actually pretty close to yours. Slightly higher BC and MV (147 ELD-M at 3100 fps from a 6.5 SAUM), so the error would actually be less in reality. And yes, a 34 degree slope is pretty dang extreme too, so shooting at such an extreme angle would introduce shooting difficulties not even represented in the numbers. Here again, I chose an extreme angle so reality would most likely be kinder, not worse, and to make any errors large enough to be visible.