At Appleseeds across the nation, we work with our students to zero their rifles at 25m (82'). This is done for many reasons, not the least of which is the suitability of this distance in how it correlates to an effective Battle Sight Zero for many, many rifles. Usually without any further adjustment, a military pattern centerfire rifle can take a 25m zero onto the KD range, and ring steel out to about 300 yards.
That's all well and good, but what about your scoped .30-06? Or how about any number of different rifle/ammo/sight combinations that would invalidate the 25m zero? Or, what if you aren't using military-spec ammunition?
There is a simple, yet effective means of determining a BSZ for *any* rifle/ammo combo, and I will attempt to explain that procedure below.
Start by establishing an exact 100yd zero on the rifle (meaning POA = POI) *DO NOT USE 6'oclock hold! (explained at the bottom)
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, I will add some numbers to the above: (The following is based on the amalgam "Rifleman's Trajectory" of 3,3,3,4 )
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 AS standard 20" target.
In order for our round to be effective against a 20" target, 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, our resulting impacts should be thus:
100 + 2 @100 = +2
100 + 2 @200 = -2
100 + 2 @300 = -9
100 + 2 @400 = -19
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 at somewhere between 300 and 400 yards. Not bad, right?
Why not keep going? Why not add more? Because of Maximum Ordinate (MO). 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. Without consulting ballistics tables and whatnot, it is extremely difficult to determine the MO of a particular rifle/sight/ammunition combination.
BSZ should be your rifles default sight setting, 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 500yards, 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.
I have used the above process for establishing a BSZ with a number of different rifles (20" iron sighted AR-15, scoped 7.62x54r PSL & iron sighted .303 British) and found that it worked exceptionally well.
If you have the time, inclination & the facilities to do so, please test out the above with your rifles, and report your results. 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.
Now, why not use 6 'o clock? 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 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 (as long as they are large enough to see, of course). Personally, I use 4MOA squares for this procedure.
Keeping an M1 or M14's sights at BSZ or anything else other than bottomed out is a good way to bend a rear sight. The first time that you go to field strip it and use some pressure to remove the trigger group without bottoming out the sight you will discover why you want the rear sight set to be bottomed out.
You can do the exact same thing that you suggest, and then set the sight to zero elevation with these rifles. In the case of my M14 my bsz is 8 clicks with South African (1991). For my Service Grade M1 it is 11 clicks with HXP69. Pull it out of your safe, count up x clicks and go to work.
It is also important to note that different ammunition can have different BSZ. Know what you are shooting!