Newbie Ballistics Chart Question

[TacticalBoltKnob]

So, I will soon be getting into longer-range shooting with my new .308 bolt gun once the weather isn't so terrible. I have been shooting for years and have been ravenously studying the long range discipline for the past year or so.

But one thing continues to escape me.

So, the proper thing to do with any new load is to verify velocity, zero your gun at 100 yards (or meters) and then work up some ballistics charts for bullet drop, etc., based on varying conditions.

But... Let's say I zero my gun and test velocity with a specific load at 40 degrees and a specific barometric pressure. Then let's say the next time I shoot, it is 30 or 40 degrees hotter and slightly different conditions all around. Wouldn't my 100-yard zero be slightly different (higher POI) and my velocity be higher and hence, skew all suqsequent, longer distances?

So my question is this: How can any bullet drop charts be accurate if you are zeroed for specific conditions. Wouldn't you have to re-zero your rifle every time you go out, for those conditions, for your charts to be spot on?

I have to be missing something simple, but nonetheless, this bothers me greatly. Any help is appreciated.

 

[shovelstrokeed]

No, your not wrong.
That's why ballistic apps like Shooter allow you to input a variety of temperatures when calculating bullet drop.
They do provide some measure of correction for powder temperature and air density that way. Mostly air density.
Since powders can vary greatly with their reaction to temperature and even that changes depending on the temperature of the chamber and how long it takes you to send the round after closing the bolt, it is difficult to predict exactly where the next shot is going to land. Science can only take you so far.
You can make some estimates and fine tune your velocity corrections at different air density and thus develop a more comprehensive data book than just a ballistic calculator will give you. Most of the calculators, unless told to act differently will compute a density based on STP conditions (standard temperature pressure) Even that varies according to which STP version you select. The ones I have seen are based on 59 deg F and 1 Atmosphere of pressure. My work, which is in chemistry, STP is 0C and 1 bar of pressure. That is a difference of 27 deg F and 10mm of pressure. Things get crazy from there.

Given all of the above, I have found that Shooter and JBM give pretty accurate drops when I use them changing little from air temp and muzzle velocity estimates based on info from reloading manuals. When you convert temperature to deg K which is what I do, small variations in temperature become pretty real in ultimate impact changes.

 

[TacticalBoltKnob]

I understand all that, but that still doesn't account for the change in zero that you will see during different atmospheric conditions. If your 100-yard zero is .5" higher or lower, then all your subsequent drops at 200, 300, 400, etc, will be off because the tables are based on your round hitting dead-nuts bullseye at 100. Therein lies the dilemma. If all ballistics calculations rely on the fact that your 100 yard zero will be perfect (when it never is, unless you are shooting in the exact same conditions as when you zeroed), then how can you rely on them? A shot that is .5" low at 100 is going to have a pretty big effect at 1000. Or am I over-thinking it and this is simply the reason that people are simply happy to hit a large-ish target at long range?

Would a solution be to try to zero your gun when it's not too hot and not too cold, say in the 45-75º range, so that your 100-yard zero variations are kept to a minimum?

 

[jasonnorcal]

I was under the impression that at 100 yds the bullet is affected very little by external factors, like percentages. That's why you zero at 100. I think your idea of going on a average temperature day is great.

 

[shovelstrokeed]

Of course you can't discount density altitude or air density factors in your calculations. You can see them in a ballistic calculator very easily. I preset mine before I leave for the range based on weather forecast for the actual location I'm shooting. I already know which ammo I'm going to shoot and have variations in high to low muzzle velocity accounted for, in fact it is easy with JBM or Shooter or, for that matter Ballistics AE, all are free Apps that work quite well on my iPhone, iPad or iMac. JBM is a little flakey on Safari so I run it on Firefox on my iMac when playing at home.

I then print out a range card for that day and I'm pretty much all set. My rifle is easily capable of 1/4 MOA groups, me, not so much. More like .75 MOA at either 100 or 200 based on averages. I tire easily and the concentration required for consistent small groups is difficult to maintain. As things move out, staying dead nuts zero gets harder and harder. Center of group may be dead nuts but not a single round in the x-ring is not uncommon. I generally use the pasters that come with Shoot n' See targets as an aiming point. They are a little less than 1" in diameter. Going 4 for 5 or even 5x is easy at 100, less so at 200 and, for me, unlikely at 300. I'm not unhappy about a 1.5" group at 200 and downright pleased with a 1" group.

I use a Mil/Mil scope and so 1 click on mine is 0.1 mils or 0.36" at 100 yards. On a MOA turret, generally, 1 click is 0.25" at 100. Numbers are not exact but rounded a bit for simplicity. It isn't hard to be off a significant fraction of a click and be outside of that x-ring. In that sense, you are really over thinking it unless you are shooting at ring targets for score or just for group size. On hit or miss stuff like steel plates you have a lot of margin and it doesn't matter.

 

[nuclear_shooter]

Of course you can't discount density altitude or air density factors in your calculations. You can see them in a ballistic calculator very easily. I preset mine before I leave for the range based on weather forecast for the actual location I'm shooting. I already know which ammo I'm going to shoot and have variations in high to low muzzle velocity accounted for, in fact it is easy with JBM or Shooter or, for that matter Ballistics AE, all are free Apps that work quite well on my iPhone, iPad or iMac. JBM is a little flakey on Safari so I run it on Firefox on my iMac when playing at home.

I then print out a range card for that day and I'm pretty much all set. My rifle is easily capable of 1/4 MOA groups, me, not so much. More like .75 MOA at either 100 or 200 based on averages. I tire easily and the concentration required for consistent small groups is difficult to maintain. As things move out, staying dead nuts zero gets harder and harder. Center of group may be dead nuts but not a single round in the x-ring is not uncommon. I generally use the pasters that come with Shoot n' See targets as an aiming point. They are a little less than 1" in diameter. Going 4 for 5 or even 5x is easy at 100, less so at 200 and, for me, unlikely at 300. I'm not unhappy about a 1.5" group at 200 and downright pleased with a 1" group.

I use a Mil/Mil scope and so 1 click on mine is 0.1 mils or 0.36" at 100 yards. On a MOA turret, generally, 1 click is 0.25" at 100. Numbers are not exact but rounded a bit for simplicity. It isn't hard to be off a significant fraction of a click and be outside of that x-ring. In that sense, you are really over thinking it unless you are shooting at ring targets for score or just for group size. On hit or miss stuff like steel plates you have a lot of margin and it doesn't matter.

What I recommend is to use a powder thats relatively temperature stable, and then make ballistic charts with range on the Y axis and density altitude on the Y axis. JBM has an app that allows you to do this: JBM - Calculations - Trajectory Card. Then you read of DA from your Kestral, this will take into account basically all of your weather variables (temp, humidity, barometric pressure/altitude). No need to have a bunch of different charts for different conditions. Only issue is if your MV varies with temperature (due to temperature sensitive burn rates), your chart will become more complex.

 

[TacticalBoltKnob]

Thanks, guys. I guess it is just something that you have to account for as best you can and hope it doesn't affect things too much.

 

[CoryT]

At 100 yards, I only need .02 mils of adjustment to go from -1000 DA to 15,000 DA. So, with a 100 yard zero, the change has no effect that needs to be accounted for in any way. The problem would only be noticed with longer range zero's, say 300 yards or 500 yards, and it's still pretty small. Zero at 300 with a 175gr .308 at 3000' DA, then move to 15000' and the shift is .1 mil, if you dial up your correct elevation under current conditions for a 900 yard shot, you'll only be off .1 mil, which is about the average hold error anyway.

Now. if you zero at 3000' DA at 100 yards, get DOPE to 900, then try to use that setting when you are at 15,000'DA, things are not going to work out well. At 3000, you need 8.3 mil, at 15000 you only need 6.8.

 

[TacticalBoltKnob]

At 100 yards, I only need .02 mils of adjustment to go from -1000 DA to 15,000 DA. So, with a 100 yard zero, the change has no effect that needs to be accounted for in any way. The problem would only be noticed with longer range zero's, say 300 yards or 500 yards, and it's still pretty small. Zero at 300 with a 175gr .308 at 3000' DA, then move to 15000' and the shift is .1 mil, if you dial up your correct elevation under current conditions for a 900 yard shot, you'll only be off .1 mil, which is about the average hold error anyway.

Now. if you zero at 3000' DA at 100 yards, get DOPE to 900, then try to use that setting when you are at 15,000'DA, things are not going to work out well. At 3000, you need 8.3 mil, at 15000 you only need 6.8.

Cool, thanks.

I guess it is more of an ammo temperature thing. I believe the pressure starts ramping up pretty steeply once you get above 80ºF, obviously affecting your velocity ... and in turn your zero.

 

[M40_A1]

If I may interject, one thing along these lines that I've always wondered is regarding "altitude density". If you know the local barometric pressure, why do you also need to input the altitude? Or do you?

 

[justinbaker]

If I may interject, one thing along these lines that I've always wondered is regarding "altitude density". If you know the local barometric pressure, why do you also need to input the altitude? Or do you?

If you have the actual station pressure from a device (ie kestrel or GPS) , then no you don't enter altitude.


If all you have is the corrected pressure then you enter altitude



Eta: when I choose absolute pressure, the altitude is grayed out and it will not let me enter anything in that Field

 

[M40_A1]

If all you have is the corrected pressure then you enter altitude

Thankyou. So, when you say "corrected pressure", does that mean corrected to sea level, and that is why you would then need to enter the altitude?

 

[farmerbrown]

Yes, then it will then try to use altitude (and temp,etc) to estimate the air density using models for the atmosphere.

Sent from my iPad using Tapatalk HD

 

[nuclear_shooter]

Cool, thanks.

I guess it is more of an ammo temperature thing. I believe the pressure starts ramping up pretty steeply once you get above 80ºF, obviously affecting your velocity ... and in turn your zero.

Depends on the powder you use. You'll have to test/research yourself.