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Posted: 5/7/2007 11:14:58 AM EDT
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Please don't laugh me off the site for asking such a novice question as what follows: O. K. I took my new Bushmaster M4 carbine to the range yesterday and followed the Company's instructions for zeroing the weapon at 25 meters. That's done, but my question is, how do I use a 300 meter zero when I am shooting at 100 yard targets? Do I manipulate the elevation dial to engage 100 yard targets? Thanks for any help you can offer. |
| Turn the rear dial all the way down so that the 8/3 or 6/3 marking (whichever dial you have) is on the left of the rear sight housing & centered. (When you do this, you'll know what I mean). As long as you zeroed properly, than that zero is good from 0 to 300 yards, no need to mess with anything when shooting between those ranges. Only when you go past that do you turn up your rear elevation. |
It's 'Good' if your targets are 40" tall by 18" wide and you are only concerned about putting a bullet somewhere on the target. At 100y the military zero will put the bullet about 6" over the point of aim, it will be even higher at 200y. The best option IMHO for zeroing is at 50yards for a nice flat 200M zero where the bullet will stay on a beer can from muzzle to about 250y. If you have an adjustable elevation wheel (which you do) you can follow the Improved Battlesight Zero which will allow you to zero your sights so that you can have a nice flatt 200M zero, yet still dial in the ranges as show on your elevation wheel. |
Well said & you are right. I was giving instructions for viable center mass shots only for a mil zero. If you want to drill tacks, than you have to do a lot more tweaking. 
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Forest, my rifle has a fixed A2 carry handle and my elevation wheel begins with 8/3. Using the IBZ, how can I still dial in the ranges as shown on my elevation wheel if the lowest 8/3 setting equals 200 instead of 300? |
(Not Forest) I'll post some detalied pictures showing how to accomplish this tomorrow. (You need to change your elevation drum so that it goes lower thean 8/3) |
There is no 100 yard setting on the elevation drum, nor is there a 100 yard setting incorporated into the Improved Battlesight Zero. However, it can easily be accomplished. I'll post the details along with the pics tomorrow. |
| Molon and Forest, I already loosened the allen screw under the rear aperture and adjusted the elevation wheel so that it bottoms out at 3 clicks below 8/3. This gives me a 50/200 zero which is recommended by Lt Col Santos. The only thing I don't understand is how can I continue to dial in the ranges as show on my elevation wheel. Do not the other numbers on the elevation wheel become meaningless, since 8/3 now equals 2 instead of 3 on my rifle? |
| My rifle sight is adjusted so that it bottoms out at 3 clicks below the 8/3 mark. Bottomed out, it is zeroe'd at 200. If I move the sight up 3 clicks so that the 8/3 mark is lined up, I am zeroe'd at 300. I can also leave the sight set at 8/3 and flip up the large aperature 5mm sight which is marked 0-200 and be zeroed at 200. The only problem is that this sight is not as precise as the 1.75mm long range aperature. |
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RIBZ Revised Improved Battlesight Zero The standard A2 rear sights on an AR-15 were designed with elevation settings for 300 to 800 meters. The Improved Battlesight Zero allows for an elevation setting of 50 yards/200 meters for one of the most all-around useful trajectories obtainable with the 5.56mm/223 Remington cartridge. Neither of the above sighting schemes allow for an elevation setting giving you point of aim equals point of impact at 100 yards. Since 100 yard shooting ranges are one of the most commonly found ranges in the United States it would be useful to have such a setting on our AR-15s. This can be achieved quite easily with nothing more than a 1/16” allen wrench. It’s really just a matter of taking the Improved Battlesight Zero one step further. There is a witness hole on the A2 rear sight base that gives you access to the index screw that you will need to loosen with the 1/16” allen wrench to make the necessary alteration to the elevation drum. The witness hole lines up with the index screw only when the elevation drum is set to 8/3. Set your elevation drum on the 8/3 setting and turn the screw counter-clockwise 3 to 4 complete turns. You don’t want to remove the screw. You want to loosen it enough to allow the elevation index wheel (the top part of the elevation drum with the numbers on it) and the elevation knob (the bottom part of the elevation drum) to separate and be able to move independently of each other.      Once you have loosened the index screw, use one hand to hold the allen wrench in place on the screw and with your other hand turn the elevation knob (the bottom part of the elevation drum) 4 clicks clockwise. You should see the rear sight base moving up while you do this. The elevation index wheel should not move while you are doing this. (We really only need to turn the elevation knob 3 clicks clockwise, but by turning it 4 clicks we get a “buffer zone” that keeps the rear sight base from coming into contact with the lower receiver when the sight is moved to the lowest used setting. This gives you a more consistently repeatable sight setting.) Now that you have moved the elevation knob 4 clicks clockwise, taking care to make sure the elevation index wheel has not moved, firmly tighten the index screw. Starting with the elevation drum at the 8/3 setting, you should easily be able to turn the drum counter-clockwise 3 clicks and see the rear sight base moving down. (You should be able to turn the elevation drum one more click counter-clockwise for our “buffer zone.”) You are now ready to head to the shooting range. As I mentioned earlier, this sighting scheme is really just taking the Improved Battlesight Zero one step further. Starting with the elevation drum at the 8/3 setting, turn the elevation drum counter-clockwise 3 clicks (8/3, -3). This is your 100 yard point of aim equals point of impact setting. From a distance of 100 yards, zero the rifle with the rear sight at this setting. Do not change this rear sight elevation setting while zeroing. Use the front sight post only to make changes in elevation while zeroing. Once you have zeroed the rifle with this method, you will have your 100 yard zero setting and the other settings of the elevation drum remain intact (within the limits of the coarse adjustments of the A2 sights). To use the Improved Battlesight Zero, simply set the elevation drum to 8/3, -2. To use the standard military 300 meter setting set the elevation drum to 8/3 and so on. Obviously, slight variations will be encountered depending on the ammunition used and the actual amount of movement with each click of the various sights. The results will certainly be close enough “for government work.” When using this sighting method, you may notice that once you set your 100 yard zero, the base of the front sight post is starting to rise above the well in the front sight base. If this happens, the “0.040” taller” front sight post from Bushmaster provides a quick and inexpensive fix.  100 yard zero (8/3, -3)    Improved Battlesight Zero (8/3, -2)    Standard 300 meter zero (8/3)  |
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The idea of using a 300 meter zero has to do with “point blank range”. The idea is that if you zero at 300 meters you will be able to aim center of mass at a person anywhere between 0 and 300 and hit them. This puts you exactly on point of aim around 25 meters and at 300 meters. It is not exactly 25 meters where the bullet first intersects the line of sight. That is why you have to set your sight to 8/3 +1 and then turn it to 8/3 when you are ready to fire. The M4 does have near perfect intersection at 25 and 300 and there is no need to turn the bullet drop compensator (BDC aka rear sight) after zeroing at 25 meters. The idea of point blank range is something hunters also use. You zero your rifle at a specific range. If you want to hit within a 6’’ sized area on an animal you calculate how far and how close you can be from you zero range and still accomplish this. This is ammo, barrel length, environmental, specific but it will give you some idea. With a 300 meter zero your biggest deviation from point of aim will be at 150 meters. Note this is halfway between 0 and 300. At this range the bullet will impact approx 6-7’’ above your point of aim. If you zero at 200 yards your biggest deviation will be at 100 yards where you will be about 2’’ high. If you don’t adjust your sights and shoot out to 300 meters you will be about 9’’ low. The advantage to this is if you adjust your sights as molon described you can easily turn the BDC to 300 meters. Another option if you don’t want to change your sights or use an aimpoint is to zero at 250 yards. This is what I prefer. Using a 250 meter zero your biggest deviation above point of aim is at 125 meters where you would be about 3’’ high. But at 300 meters you would be about 4’’ low. Basically it does the same thing as a 300 meter zero but flattens out the arc of the bullet. The trade off is that instead of having an exact point of aim, point of impact at 300 meters you will be a little low. I accept the tradeoff because most shot are less than 300 meters. It also makes calculations easy to do in your head. With a 250 meter zero I use the following 50-225 aim four inches low 225-275 aim dead center 275-300 aim four inches high Using this system I will never be more than 1.5’’ off at any range up to 300 meters and I don’t have to adjust the sight. You just have to see how far you plan on shooting and what system will work best for you. If you use an aimpoint the .223/5.56 starts to drop pretty quick after 350 meters making hold-overs difficult to use. Even if you have a BDC the wind begins to have a big effect and needs to compensated for beyond 300 meters if it is over 10 mph or so. |
| Marine Corps Doctorine teaches us to use the 300 meter dope for anything 300 meters and under. I agree with the above posts that a 200-250m dope is better for more accurate shots under 300 meters. I was never allowed to modify the sights on my issued rifles or carbines but when Marines qualify at the rifle range we shoot at 200, 300 and 500 yards (some ranges are set up for meters). On an A2 upper 8/3-2 is the standard dope used for the 200 yard/meter line. This is the dope I usually have on my rifle (not the "standard" 8/3). This allows for a good 200 meter dope and still lets me adjust my sights for longer ranges if needed. I'm not trying to knock on the posts above me as I'm sure they work well, just wanted to let you know you could still dope your rifle for 200 meters without modification to the sights. |
| It only works fine if your target is big enough. If your target is 10'' or more than you wont have a problem. If you are trying to hit a 1 moa or 2 moa target you will have to make changes to your sights. It is just another option. If you can hit what you aim at with the way your sights are set up then you are right. |
| I learned using the 300m zero on an A2, and so that's what I use on my A2. I've shot them that way enough that I know where the rounds will be a bit high, and compensate. I don't really like the idea of fiddling with my sights, but everyone will have their own preference. |
| A 300 meter zero will be about 6" high at 200 meters and about 6" low at 350 meters. This is good enough for a center of mass hit on a 40" tall by 18" wide E type target out to 350 meters. If I need to shoot at smaller targets at closer range, say clay pigeons at 100 meters, I will drop the rear sight 3 clicks. If I need to hit large targets (E type or IDPA) at up to 200meters, I will use the 5mm large aperature sight. If I need more precision at long range I can always mount a scope. |
Talk about a  post.The IBSZ was meant for combat shooting - not X ring shooting as LTC Santose pointed out. With a standard Military 300M zero it's VERY easy to completely miss a target that is using cover or down in the prone posiiton if you aim COM. With the IBSZ you continue to aim COM but have a precision of +/-2" instead of -2"/+8" out to twice typical combat ranges. If all you do is qualify on a pop-up course, then the standard Army 300M will work fine, If you want to be able to hit targets that using cover you are better served with the IBSZ. |
That is incorrect. 8/3 is still 300M using the small aperture. 200M is 2 clicks BELOW the 8/3 point using the small aperture. Only the larger aperture on the 8/3 setting (it should approximate the same POA/POI as the small aperture on the 8/3 -2 setting). |
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