ballistic table trouble.

[g20gunny]

I am getting conflicting results with all of my ballistic calculators with this particular load... I know my bc, know my velocity and zero but somehow my calculators keep giving me consistently incorrect elevations. I am putting in all the correct information including altitude, humidity, temp and pressure, but somehow my rifle consistently shoots differently. I don't mind if it's a little off but I'm wasting time and ammo walking them in when my calculator gives me wrong information. I even had another professional shooter calculate my elevation and at 1100 yards he was 6 minutes high.
I am shooting a 300 win mag with 208gr Hornady match with a .62bc at 2984 fps at 6300 altitude. I know my zero is at 100 yds, 16.75 minutes at 850 yds, 21 minutes at 1000, 26 minutes at 1100. These figures may be a TINY bit off but when I dial my figures in I can get first shot hits but with the calculators I have to walk them back several minutes. Should I just put in a fake bc untill I get a table near what I am getting ot just make my own table and carry it with my rifle? Id sure like to be able to type in a range into my calculator and be at least close to my target.
Any input or similar experiences would be appreciated.

[zoomie]

AMax or BTHP?

Exbal shows BC of the AMax bullet of .648.

Try Bryan Litz's BC on JBM. He has G7 of .311 for the BTHP Match and .324 for the AMax. See if that works better.

http://www.jbmballistics.com/cgi-bin/jbmtraj-5.1.cgi

[zoomie]

double

[g20gunny]

It's a bthp... ran the link and it was near dead on my calculations! Thank you zombie for the link! Seems like a great calculator that I can run off of my phone.

[emcon5]

Make sure you include the correct sight height.

[g20gunny]

Yeah I think the sight height is good, used my caliper from my tube dia/2 to the center of my firing pin hole. I'm sure there are better methods but that was what I could come up with.

[Hummer70]

When I was at Aberdeen Proving Ground I got the skinny on ballistic programs from the boys at BRL (Balllistic Research Lab).

The rule of thumb is down range ballistics trajectory match calculations 5% of the time and may be correct at say 300 and 700 yards but not so at the other ranges.

Best thing to do is after you arrive at your load make up a 4X8 ft. frame from 2X2 wood. Cover it with cardboard (fridge box, mattress box etc) and then cover it with news print. (your local paper will generally sell end rolls of news print for very cheap.

Zero your rifle at 300 yards and erect the frame and put a target up at the top of the frame. Cover it with news blanks news print. Shoot 3 shots.

Without touching scope knobs move back to 400, 500, 600 yards or further and shoot three rounds.

Measure the center of the holes from the center of the target at the top and record and average the drops.

Only then will you have the drops from your rifle in your weather conditions and altitude and your data will be pretty solid. Even better shoot 10 rounds at each range.

We did it a little more scientifically at Aberdeen, three rifles were selected from a test selection that came it. We got the best shooter, a median shooter and the poorest shooter (as long as it met acceptance accuracy) and fired them and two Master Class Shooters fired 3 sets of 10 at each range. Bottom line each rifle got 60 rounds at each yard line times 3 rifles so we had 180 rounds of data.

The target was divided into quadrants with a vertical and horizontal line running through the point of aim. (This would be 6 o'clock bottom of black on round bull target). We measured from the horizontal line to the middle of the bullet hole in millimeters. Shots to right of vertical center line and below horizontal were called minus plus hits meaning it was below the horizontal and to the right of the vertical line. Conversely shots low left quadrant were called minus minus.

All this data was fed into a computer program which would calculate the mathematical center of each group and the average drop in MMs. the computer would identify the center of group for each rifle and the average center for all three rifles. Fascinating information.

This is why the military ballistic tables for ammo are "on the money" and not done by some program.

But assuming non one can afford 30 rounds at each range three will generally do it for you.

[tahunua001]

what's your sight height? that is a major thing to consider with ballistics calculators.

[RaySendero]

g20gunny wrote:

I am getting conflicting results with all of my ballistic calculators with this particular load...

I am shooting a 300 win mag with 208gr Hornady match with a .62bc at 2984 fps at 6300 altitude. .....

I'd sure like to be able to type in a range into my calculator and be at least close to my target.

Any input or similar experiences would be appreciated.

gunny,

If your always shooting low relative the calculated G1 trajectory at long range, Look up the G7 ballistic coefficient for that 208gr Hornady match bullet and rerun your numbers.

[Bart B.]

In my experience, sight height above the bore axis is the least important part of making ballistic calculations. At any range and not using it in calculations, the error will always be equal to sight height. If sight height is 1.5 inches, the error's 1.5 MOA at 100 yards. At 1000 yards, the error is 0.15 MOA.

I've shot four different bullet weights (155, 180, 190 and 200) from .308 Win. cases getting zeros at 100 yards for each. Then moved the sight down so the group would center below point of aim a distance of sight height plus bullet drop. That put the line of sight parallel with the line of fire as I shot the rifle from prone; a true, exact bore sight setting on the sight. I recorded the atomspheric conditions for temperature, humidity, etc.

Sierra Bullets' older ballistics software was used to calculate sight settings for all four bullet weights. Each load had been previously chronographed so I had muzzle velocity data. Sight settings were calculated for 300, 600 and 800 yards for different atmospheric conditions expected at the range. Firing tests at each range showed calculated sight settings were within 1/4 MOA at 300 yards and 1/2 MOA at 600 and 800 yards.

[kraigwy]

Do a box test with your sights. Get a large target. Shoot the target where its zeroed at the bottom left. Then click up 5 min shoot again. Check and see if your scope really clicked up 5 min. Then click up 5 min more. Shoot, then click 5 min right, shoot again, click 5 more right, and shoot. Then click 5 min down and shoot. Click 5 min down again and shoot. Now do the same thing, click left 5 min and shoot, 5 more and shoot.

You should now be back to your zero. But I bet you aren't. Few scopes are.

If you're shooting 1000 yards (for your max distance. on a large target take the come-ups your BC programs call for, and do the same box test taking the scope to the highest elevation you want to shoot. I bet after you do the box test you wont be at your zero.

Scopes get their click measurements by the threads in the dial. X threads per inch. People assume that that's what their scope will do. Even expensive scopes don't always.

Plus the scope is probably canted a bit, sure you leveled it, but its near impossible to get it perfect.

That is why BC programs are guides. They get you on paper but you need to get a zero for every range you plan on shooting and write it down.

The programs are like reloading guides, they say with this and that, you'll get this results. But I never got the exact results from a load that compares to what the Reloading Guide says. That's why they call them guides.

Use the BC program to get you close. Shot to confirm, the record what you actually get at the range on a data card and use that.

BC programs are like dealing with Russians, Trust but Verify.