Author Topic: Marksmanship: Battle Sight Zero, or Prairie Dog Zero (July - August, 2013)  (Read 3988 times)

Offline Nero

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Marksmanship:  Battle Sight Zero, or Prairie Dog Zero
by ItsanSKS and Nero

So what's a "Battle Sight Zero," and why should you care?  As you might guess by the name, this is another concept that Appleseed borrowed from our military.  If you find yourself on a two-way shooting range, there's no time to do a careful range estimate, consult a come-up table, dial in the appropriate number of clicks, and then take the shot.  Instead, you want one sight setting that's good enough to score a quick hit from point blank out to a reasonable range--that's your Battle Sight Zero.

At Appleseed shoots across the nation, we work with our students to zero their rifles at 25m (82').  One of the reasons for using this range is that  it correlates to an effective Battle Sight Zero for many rifles, particular those derived from military designs.  Usually without any further adjustment, an M1, M1A, or AR-15 with military standard ammo and sights can take a 25m zero onto the range and ring steel out to about 300 yards. 

That's all well and good, but what about your scoped .30-06 deer rifle?  Or, what if you aren't using military-spec ammunition?  Or how about any number of different rifle/ammo/sight combinations that would invalidate the 25m zero?  Or maybe you're shooting a rimfire at smaller game, what then?

There is a simple, yet effective means of determining a BSZ (Battle Sight Zero) for any rifle/ammo combo, which is not only useful in itself, but is a demonstration of the minute of angle and trajectory concepts taught at an Appleseed known distance (KD) or full distance shoot.


The principle of BSZ is simple:  You care about scoring an adequate hit on a given target at a variety of distances.  Since the target size remains fixed, its effective angular size becomes greater as the range is reduced.  For instance, the nominal human torso size of 20" is 5 minutes of angle (MOA) at 400 yards, but a full 20 MOA at 100 yards.  We'll need to pick a zero that is fairly precise at the far range, but we've got a lot of slop to play with closer in.  By determining the trajectory of your ammo/rifle/sight combination, you can customize a BSZ (or prairie dog zero) for your situation. 

(Notes:  For the following exercise, use a center of target sight picture, not a "6'oclock hold".  We'll explain why later.  The following exercise is stated for centerfire rifles.  If you have a rimfire, collect your data at 25, 50, 75 and 100 yards - we work out a 22LR scenario below.)

Start by establishing an exact 100yd zero on the rifle, with point of aim (POA) matching point of impact (POI).  Do not change this zero throughout the exercise.
Shoot at 200 yds with the 100yd zero.  Record the difference in POI vs POA.
Shoot at 300 yds with the 100yd zero.  Record the difference in POI vs POA.
Shoot at 400 yds with the 100yd zero.  Record the difference in POI vs POA.
Shoot at 500 yds with the 100yd zero.  Record the difference in POI vs POA.

For illustrative purposes, we add some numbers to the above:  (The following is based on the averaged center-fire "Rifleman's Trajectory" with 'come-ups' of 3,3,3,4 MOA)

100@100 =  0"
100@200 = -6
100@300 = -15
100@400 = -27
100@500 = -47

Next, we need to know the size of the target we wish to be able to 'effectively' engage with our BSZ.  Let's keep with the Appleseed 20" KD target, which is copied from the military standard.  In order for our round to be effective against a 20" target, if we are using a center of target hold, it can neither rise, nor fall, more than 10", and less is better!

Looking at the numbers above, we can already see that our 100yd zero does pretty good against the 20" target, falling only 5" too low at 300.  We can do better than that!  If we add 2MOA to our 100yd zero--remembering that 1 MOA is 1" per 100 yards--our resulting impacts should be thus:

100 + 2 @100 = +2  (we added 2" = 2 MOA)
100 + 2 @200 = -2  (we added 4" = 2 MOA)
100 + 2 @300 = -9  (we added 6" = 2 MOA)
100 + 2 @400 = -19  (et cetera)
100 + 2 @500 = -37

1" inside the target @ 300 isn't very comforting, as that's just 1/3 MOA! Can we do better?  YOU BET!  Lets add another MOA to our initial 100yd zero, and see what that does for us:

100 + 3 @100 = +3
100 + 3 @200 =  0
100 + 3 @300 = -6
100 + 3 @400 = -15
100 + 3 @500 = -32

It looks like setting our sights to be 3" high at 100 yards (in essence using a 200yd zero), will give us a nice BSZ, capable of making an effective hit out to somewhere between 300 and 400 yards.  Not bad, right? 

Why not just keep going?  Why not add more?  Because of Maximum Ordinate (MO) - the greatest height above our sight line which is reached by the bullet as it travels down range.  Changing the departure angle of the round is going to affect how high the round eventually gets above the line of sight before it starts coming back down.  We don't actually know where that is, since we only checked a few distances.  Without consulting ballistics tables and whatnot, it is extremely difficult to determine the MO of a particular rifle/sight/ammunition combination.  So we need to be conservative on the 'up side' so that our trajectory won't rise above the target at some intermediate range: Real targets in the field don't appear at neatly measured distances!

BSZ should be your rifle's default sight (or Base Sight) setting.  That is to say, that whenever you put your rifle into the safe, it's sights should be set to its BSZ.  This enables the Rifleman to grab his rifle and engage targets out to ~300yd  without even touching his sights.  If he knows the target is beyond 300, say at 400 yards, it would be a simple matter of dialing in an additional 3 MOA to his sights.  To engage a target at 500 yards, starting with the BSZ above, the Rifleman would need to add only 6 MOA to his BSZ.  In essence, the Rifleman would then only have to remember two numbers: 3@400, 6@500. 

ItsanSKS has used the above process for establishing a BSZ with a number of different rifles (20" iron sighted AR-15, scoped 7.62x54r PSL and iron sighted .303 British) and found that it worked exceptionally well.

If you have the time, inclination, and the facilities to do so, please test out the above with your rifles, and report your results as a comment here.  For instance, we already know what the M1's BSZ should be- 200yd zero +2 MOA, resulting in a 275yd BSZ. Let's see how close to that the above process gets us.

So much for the redcoats; how about the deer and those prairie dogs?  For those, you can use an even more generic form of the BSZ procedures.  To begin, you need to know the size of the critical area of your target, plus the span of distances at which the target may appear.  Do the trajectory exercise above, zeroing at the closest likely range, and working in steps out to the longer range of interest.  Match the bullet drop against the size of your target, and adjust sight MOA following the plan above.  (You will soon find out why "flat shooting" rounds are prized.)

Now let's try the exercise for a rimfire round.  Suppose we're plinking those prairie dogs.  Let's say we have a 10" size on a standing dog, and they may appear at ranges from 25 to 100 yards.  Nero ran some Federal Automatch through his scoped Ruger 10/22T and collected the following data after zeroing at 25 yards:

25@25 = 0"
25@50 = -.25
25@75 = -2
25@100 = -7

Will this zero work?  No.  If you're using a center of mass hold, the bullet will hit the dirt before it reaches Mr. Dog out at 100 yards: It's down 7 inches, and half the target height is 5".  How about we try adding in 4 MOA?

25 + 4 @ 25 = +1"  (4MOA = 1" at 25 yards)
25 + 4 @ 50 = +1.75" (4MOA = 2" at 50 yards)
25 + 4 @ 75 = +1"  (4MOA = 3" at 75 yards)
25 + 4 @ 100 = -3" (4MOA = 4" at 100 yards)

Much better.  All of those numbers are within the required plus or minus 5 inches.  So if I zero at 25, and then come up 4 MOA (16 clicks on most scopes), that's a prairie dog zero with this rifle and round.

Finally, why not use a 6 o'clock hold?  Let's say you're zeroing on a 4" target @ 100yds.  If you hold a perfect 6 o'clock and adjust your sights so that your rounds impact the center of that target, your rounds are already hitting 2" above your POA.  Change the size of your initial zeroing target, and now you have to adjust your sights again to impact the center of target (COT), and you haven't even changed distance yet.  By holding COT throughout this process--for the initial zeroing @ 100yds, and doing the same for each subsequent distance--you remove this layer of inconsistency, and it doesn't matter what size targets you use (so long as they are large enough to see, of course).  We use 4MOA squares for this procedure.

This work is licensed under a Creative Commons Attribution-ShareAlike 4.0 International License by Tim Oren.
« Last Edit: March 17, 2015, 05:31:15 PM by Nero »