ballistic software question

[Nikolakangrga]

I have been looking into ballistic software programs. This may be a VERY stupid question, but I dont know the answer and you guys have always answered my stupid questions in the past. If I zero my rifle at 250 yards @ 0' elevation. Then an hour later I drive up a mountain to 3000' elevation. I punch in the distance to a steel gong I have (980 yards), punch in the wind, elevation, pressures and so forth. Will my rifle being sighted in @ 0' elevation screw up my POI at 3000' elevation? what if it were 6000' elevation?

The reason I ask, where I will be hunting black bears this year, the elevation ranges from 1000' to 4500'. I was just curious how you guys deal with the different air densities.

Thanks, Nick

 

[BryanLitz]

Nick,

As long as you zero at a short range, like 100 yards, that zero will hold for a very wide range of altitudes and atmospheric conditions.

However if you zero much further, like 600 yards, then that zero is very subject to the atmospheric conditions that were present when you zeroed.

I always advise 100 yard zeroes. They're just more reliable and simplify things greatly compared to longer range zeroes.

-Bryan

 

[texan79]

Bryan,

I'm not going to argue with you, but perhaps you could clarify for me as well. It seems that the function for entering these values into a program addresses this very issue. I'm using Shooter for android and it asks for zero conditions. (you worked on this I believe?) Am I assuming to much in thinking that it takes my base values into consideration when delivering a shooting solution at higher elevations?

 

[BryanLitz]

texan,

You're correct. In a program that allows you to define your zero conditions, if you enter those conditions properly, then your trajectory predictions will be good in all conditions.

This is a feature of the program that "allows you to do it the hard way", in my opinion.

For one thing, not all programs have that feature, so if you're using one of those programs, you simply can't use a long range zero reliably.

Second, if you used a 100 yard zero, you could simplify your process by totally skipping the steps of measuring and entering your zero conditions.

Plus, think about this. If you use a 100 yard zero, you can 'verify' and refine your zero everytime you shoot at 100 yards, regardless of the conditions. But if you're using a 600 yard zero that's subject to the conditions which you have to store with that zero, then your zero will only ever be tied to one shooting 'session'.

Again, you're correct that it's possible with some tools to overcome the challenges of long range zeroes, but it is my opinion that shooters lives would be less complicated in most cases by using 100 yard zeroes.

An exception might be a shooter who expects 'fast shot' opportunities at relatively short range, and wants to zero high at 100 yards for a ~200-250 yard 'point blank' trajectory on a given size vital zone. But that scenario is outside the scope of this 'Long Range' Forum.

Take care,
-Bryan

 

[Nikolakangrga]

An exception might be a shooter who expects 'fast shot' opportunities at relatively short range, and wants to zero high at 100 yards for a ~200-250 yard 'point blank' trajectory on a given size vital zone.

Bryan,
Thanks for the input. I completely understand what you are saying. Makes alot of sense. I put my .300WM together so it could be a point and shoot out to 300 yards, but is also capable of reaching out and making an accurate shot to xxxx yards. So in other words, Iam not entirely sure what I am going to do now. I would like to have a 250 yard zero but that may not be a wise decision.

I plugged in my info into a ballistic calculator and got this for 700' elevation and 6000' elevation:

250yd Zero @ 700' Elevation:

50yds - .8"
100yds - 2.3"
150yds - 2.7"
200yds - 2.0"
250yds - 0"
300yds - -3.3"

250yd Zero @ 6000' Elevation:

50yds - .8"
100yds - 2.3"
150yds - 2.7"
200yds - 1.9"
250yds - 0"

As you can see, at 6000' elevation my trajectory changed at 200yds. It went from +2.0 to +1.9. So if I punched into a ballistic software that I am at 6000' elevation, a perfect 1000 yard target and my rifle zero is at 250 yards. It seems as if that would work? not sure......

 

[BryanLitz]

Nik,

One possibility to consider is to establish your 100 yard zero (set your scope turrets to 'zero' there).

Then, figure out how many clicks up gives you the 250 yard zero and apply that adjustment when you're expecting a fast shot at close range. This may compromise accuracy slightly for the short range 'point and shoot' scenario (because the 250 yard zero will be slightly affected by conditions), but it will preserve the accuracy for longer range shots which is where you need it more anyway.

Just a suggestion.

Good luck with whatever you decide.

-Bryan

 

[Nikolakangrga]

Bryan,
Thanks for the input. I completely understand what you are saying. Makes alot of sense. I put my .300WM together so it could be a point and shoot out to 300 yards, but is also capable of reaching out and making an accurate shot to xxxx yards. So in other words, Iam not entirely sure what I am going to do now. I would like to have a 250 yard zero but that may not be a wise decision.

I plugged in my info into a ballistic calculator and got this for 700' elevation and 6000' elevation:

250yd Zero @ 700' Elevation:

50yds - .8"
100yds - 2.3"
150yds - 2.7"
200yds - 2.0"
250yds - 0"
300yds - -3.3"

250yd Zero @ 6000' Elevation:

50yds - .8"
100yds - 2.3"
150yds - 2.7"
200yds - 1.9"
250yds - 0"

As you can see, at 6000' elevation my trajectory changed at 200yds. It went from +2.0 to +1.9. So if I punched into a ballistic software that I am at 6000' elevation, a perfect 1000 yard target and my rifle zero is at 250 yards. It seems as if that would work? not sure......

 

[BryanLitz]

Nik,

I think you're making a common mistake for this analysis.

When you run the ballistics program with a 250 yard zero, you're forcing it to be zeroed at 250 yards. The difference in trajectory that you list above is not the actual difference you would see with a rifle that you zero at 700' and take to 6000'.

When you zero a rifle at 250 yards at 700' elevation, that 'zero' is a specific mechanical relationship (angle) between the line of sight and the bore. Now when you take it to 6000', that same specific mechanical relationship will not result in a trajectory that's zeroed at 250 yards, due to the vastly different atmosphere.

When you run the program and tell it to 'zero' at 250 yards for both altitudes, it's calculating and applying a different mechanical relationship between the line of sight and bore that results in a zeroed trajectory at each altitude (not what happens with a real rifle).

It's getting late for me, I hope this makes sense and I'm not misunderstanding your analysis.

-Bryan

 

[MontanaRifleman]

Interesting stuff Bryan. I had never considered the atmospheric affects on a long zero.

My loads are 3400 fps and I zero @ 300 yds for point blank shooting to 350 and easy hold corrections to 400 or so. When running the calc @1000' and 9000' with a 0 scope height and 25 yard zero (closest it will run) the 125 zeros are right on and the 225 zeros are 0.2" off and the 325 yd zeros are 0.6" off

I wonder what effect pitch and yaw would have on the 100 zero?

I think I'll take the 200 zeros and live with the .2" diff

Good post.

-Mark

 

[paphil]

One of the handy things taught at The Best of the West shooting school is the simple math to correct for changes in elevation and temperature. This works well for high BC bullets in pretty much big game hunting situations. It might not be fine tuned enough for prarrie dogs, but is not far off. For the first 500 yards, the point of impact won't change much at all, less than a couple of inches . AT 1000 yards, POI will raise or lower by 1/3 minute for each 1000 feet elevation or 20 degrees F temperature change. At 750 yards, it is half that or 1/6 th minute.
So if you are shooting a 7mm mag at 3000fps(berger 168, bc. 617) at 1000 feet elevation and 80 degrees and you are dead on at 100 or 200 or what ever and have developed your drop chart so it is right on out to 1000 yards. We now take that gun up to 7000 feet and 20 degrees. The air has gotten thinner because of the 6000 feet change in elevation but it also got denser due to the temperatures 60 degree drop. At 1000 yards, the elevation change is worth 2 minutes down and the 60 degree temperature will be worth 1 minute up. The net change is one minute(the gun will shoot high) or 10 inches at 1000 yards and 1/2 minute at 750 yards which is 3 1/4 inches. The temperature and altitude partially cancel each other out.
The lower the BC and velocity the greater the change so choose the highest possible BC. Run the numbers on the free calculator at the BOTW web site and have fun shooting!!