As part of the last marksmanship tip, "Know Your Rifle" (found here) a question was asked regarding how to select a zero distance for a rifle, and how to use that zero to make hits on targets at varying distances.
In this installment, I will attempt to shed some light on these processes, hopefully making your next range trip, or foray into the field more productive.
Before we get started, you have to know a few things about your rifle, the targets you hope to hit, and the expected distances you will be engaging them from.
Most importantly, as ole Harry Callahan put it, "A man has got to know his limitations."
What I mean by this, is it does you absolutely no good to have a zero on your rifle that is effective from 400 to 650 yards, if YOU are incapable of making a shot at that distance...
For the sake of this discussion, we are going to use a rifle platform that many here are familiar with, the Winchester Model 70, in .300 Win Mag with 26" Barrel.
Ammunition: Hornady 180gr SST Superformance in .300 Win Mag, which has a muzzle velocity of 3,130 fps (listed for a 24" barrel, but its close enough)
Scope: Leupold fixed 4x scope with duplex reticule, mounted 1.5" above the bore of the rifle.
Target: Caribou (assume for the sake of this discussion that the 'kill zone' on a caribou is 14")
For the sake of this discussion, we are going to also assume that the shooter is well qualified to take a shot at distances beyond 300yds, and is confident in his/her abilities to make a critical hit on this target, at 500yds.
Now, based on past forays into the field, our shooter knows that from his pre-selected vantage point, the herd of caribou will be from 300-500 yards away. Moving closer is made difficult because of terrain issues, so it would be best if the shot could be taken at this distance.
We've identified some of our need-to-know information:
Ammunition- bullet weight & muzzle velocity
Scope mounting height (measured from center of bore to center of scope)
Kill zone of our target
Expected distance to target
Now, we need to get familiar with the ballistics of our round; specifically, how much the bullet drops at varying distances. For this, we are going to use a ballistics calculator, such as the one found here: http://www.biggameinfo.com/index.asp...=/balcalc.ascx
The calculator is going to ask you some specific information, and will spit out a ballistics table based on your inputs. I have used the following inputs for the information used:
Bullet diameter: .308" Muzzle Velocity: 3130 fps Ballistic Coefficient: .480
Zero Range: 100 yards Range interval: 10 yards Max Range: 500 yards.
(bullet information provided on the Hornady website)
Go ahead and plug in the above information, at the calculator given above, so you can follow along.
Now that you've got that massive table in front of you, what to do with it?
The two important columns that you need to look at are the first two- Range (yards) and Path (Inches).
Range is self-explanatory. Path, perhaps not so much.
A negative number means that your bullet impact would be below your aiming point.
A positive number means that it would impact above your aiming point, both measured in inches.
Now, lets look at the drop information for ranges 300 through 500:
300 = -9.8"
350 = -15.4"
400 = -22.4"
450 = -29.1"
500 = -41.1"
What's this information telling us? Well, if we zeroed our rifle at 100 yards, and took a shot with it at 300 yards, our bullet would strike 9.8" below the center of our target, which would put it outside of our kill zone- (a 14" kill zone would be 7" above to 7" below) So we know that we can't zero the rifle at 100 yards and make a responsible shot on our target. Further, we know that from 300 to 500, our bullet drops a total of 31.3" (41.1 - 9.8 = 31.1") This tells us that we are not going to find a zero that will be perfect for every shot from 300 to 500 yards. However, we're not done, and I'm sure we can get a pretty good zero to work from.
Lets adjust that zero range to something more practical, right in the middle of our expected range- 400 yards. Now lets look at the table:
300 = 7.0"
350 = 4.2"
400 = 0.0"
450 = -5.8"
500 = -13.2"
That's got us right where we want to be! Take a dead-on aim at 300, and you will hit the upper portion of the kill zone. Aim dead on at 400, and you hit right where you're aiming. At 450, your bullet will impact about 6" low. A shot at 500 (the limits of our shooters ability) will be outside the kill zone. This means that all but the furthest Caribou are fair game.
To make a better shot, (one more centered in the kill zone) our shooter would aim a little low on shots nearer than 400 yards, and aim a little bit high on shots further than 400.
Now, we've determined that the rifle should be zeroed for 400 yards, for this particular hunt. He needs to zero his rifle for 400 yards, but only has a 100 and 400 yard range to work on... How can he zero his rifle, without spending GOBS of time, and hundreds of dollars in ammunition?
Taking a look back at the ballistics calculator with the rifle zeroed at 400 yards, look at the 100 yard path. It says that our bullet would impact 5.6" above our aim point, when zeroed at 400 yards... Our hunter *has* a 100 yard range... He can zero his rifle at 100 yards, but instead of knocking out the center of his target, he wants his group to be centered 5.6" above the center... Once he's done that, his rifle is (nearly) zeroed for 400 yards, and he can walk the rifle to the 400 yard line, and expect to at least be on paper at that distance, which will allow him to fine-tune his actual zero.
Point of note: I do not advocate taking a rifle into the field that has been 'zeroed' with an adjusted 100yd zero. At best, this type of zero should be used to get you 'on paper' at the actual distance you would like to use. Barrel differences, elevation differences, temperature, and all sorts of other factors come into play, and you certainly don't want to be wounding an animal.