At least a couple times a day I answer a question that goes something like this.
“ Should I just chronograph my loads and give you the bullet speed and ballistic coefficient” ?
I look at my watch and wonder if I should get into it or Just say yes to the chronograph. Well so far I have never said Yes to that question, but rather have taken the time to explain a better way.
Over the years we have used several different chronographs, all have been able to assist in load developing or testing one lot of powder to the next, but not one of them has given me the confidence in my true muzzle velocity. I have read about one chronograph that has a plus or minus tolerance of 3%. On a cartridge running 3000 fps this means you could be off 100 feet per second. On one of our 7MM Remington Magnums with a 162 grain Amax this 100 FPS translates into about 10 -12” difference at 1000 yards. Throw in an acceptable extreme spread of 15 to 20 fps and you could be off another couple inches at that range. In my opinion the chronograph serves a purpose but it shouldn’t be the end all be all for an “true” velocity.
Let me walk you through a process to getting all the right pieces of information to build a turret that will match your rifle’s trajectory. It starts with making a drop chart. Once you have your ammo shooting well in your rifle then it’s time to get out the ole trusty chronograph. Use this to get an approximate speed. A five shot average will work. Take this chronographed speed to the ballistic program at GSEVEN.COM. You will need to have an Idea of what the bullet’s ballistic coefficient is as well. ( you can look at the manufacturer's website in most cases ) Once you are at the ballistic program page you will find a place to input your chronographed speed, your bullet’s bc, and the environmental conditions where you will be shooting along with where you are zeroed ie. 200 yards. . Calculate your inputs by clicking on the calculate button located top right corner of the page. Then print off your drop chart.
Now the fun starts. Along with your rifle, ammo and other shooting gear take your drop chart.
You will need something to read the temperature and you will also need to know your altitude.
Get set up at the bench and check your zero. I prefer to zero at 200 yards. Make sure you zero wherever you set your drop chart to start from. Once verified it’s time to put up a few far targets.
I like a mid range target like 700 yards and a far target around 1000 or so. A half sheet of plywood covered in some sort of paperworks well. Now look at your drop chart, see how many MOA or clicks it tells you to dial in for that range and dial it. Adjust you parallax and shoot a five shot group. Do the same thing at your far target. Next step go check out your groups. Chances are you are high or low. If this is the case with a tape measure measure each bullet hole from center of bullseye and get an average high or low in inches. Pretty simple right? You can repeat the process just to make sure. Be sure to take note of the temperature and the altitude when you were shooting.
Step three is even easier. Now it’s time to use the handiest tools out there. Go back to GSEVEN.COM and find that ballistics program page again. You will notice halfway down the page on the right there is a tab labeled ‘trajectory validation”. Click on it. There are four areas to enter data, load data, environmental data, sighting data, and down range drop data. Enter your load data. Bullet BC, bullet weight, and your chronographed speed. Next environmental data, this is the altitude and temperature when you were shooting. It will automatically populate the station pressure and your humidity will default to 50%. Sighting data wants you to input your scope height ( center of barrel to center of scope will be adequate ), and your zero range. The is a input called zero height, this has to do with your 200 yard zero, because of different click values it sometimes inhibits you from being perfectly zeroed, thus the zero height tab. If you are as close as you can be at your zero range but still for instance .3” low enter this number into the zero height tab . use the - sign for low values. Finally the last tab, down range drop data. Enter in your far target, the number of clicks, or MOA it took at that range and then if you were high or low after you measured with your tape measure enter that value into the zero adjust tab. Click on the calculate tab. Did your velocity change? This new velocity is the true speed based on your drop and the bullets bc. Check your mid range target drop and your long range target drop to make sure the jive with each other. If the spread is huge you might need to redo the shooting. If you do, make sure your shooting conditions are in your favor. Looking for down- drafts or up-drafts or parallax , really anything that would through your numbers off.
Once you are confident in your numbers, it is time to order your turret. The BC you used in these calculations and your new calculated muzzle velocity are the two key bits of info you will need to get it ordered. You will now get a turret that actually matches your rifle’s trajectory, because you validated your chronographed speed. Sometimes the ole chronograph is right but more often than not its off.
A few years ago we made a short 15 min youtube video called Validating your rifles trajectory. You can visually watch the steps we have outlined here. On a different note there is also a way to use the G7BR2 to validate your rifle in the field eliminating the need for your personal computer. It is an awesome tool that will do more for your long range shooting than any other piece of shooting equipment out there.
Don’t take the easy way out and say yes to the chronograph. Take the time to do it right.
When you are faced with “that” shot you’ll be glad you did.