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Posted: 6/16/2012 8:10:17 AM EDT
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This is my plan today. If at any time there is a better way to do things to meet my goals other than stated, correct me. There is probably a million ways to do this and some probably make more sense than others. I want to do it the way that makes more sense than others. If there is a more effective way to do what I'm doing correct me. I'm no pro. I want a general battlesight zero of 25/300 meters for if I pick up my rifle at any given time I know that is where it will hit. I want to know where to go from there (by click count) to hit any given known distance without pulling the trigger.
I want to identify and mark where my rifle zeroes at 25 meters. I also want to identify and mark my rear elevation when zeroed at various longer ranges( 500-1000 meters). I would like to, at the end of today, be able to click to a certain elevation by counting clicks and know where my rifle will hit without pulling the trigger. OR at least damn close that I can fine tune from there. My goals today are as follows: Zero my rifle with a 25 meter zero. Identify number of rear sight elevation clicks to various known distance ranges. 500, 800, 1000 meters for example. I don't plan to shoot this rifle any UNKNOWN distances. I've got my rear elevation bottomed out. I counted from top to bottom 107 clicks. My rear is 1/4 by 1/4 My windage is centered. The base of my front post is level with the valley of the front sight(not sure the technical term) It is my understanding that this is 'mechanical zero'. I plan to zero at 25 meters, making adjustments for windage with the rear windage knob and elevation with the front sight post. Should I leave my rear elevation bottomed out for this process or should I run up a couple clicks to give myself some breathing room? I know I'm rambling. I hope this makes sense to somebody out there. I am open to ANY feedback and it will be much appreciated! |
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This type of stuff is impossible to understand unless you educate yourself about bullet trajectory. The bullet flies a path very much like a football. At short distances it appears to be going straight. As the distance increases, you will notice that the flight path curves down at an ever steepening rate. To get to longer ranges, the initial throw must have a very pronounced upward direction.
Imagine a drawing with a side view of your rifle and a downward curve coming out of the barrel, and a straight line coming through the sights. If you are zeroed at 25m, the line crosses the curve at 25m away from the rifle. The sight on your rifle is considerably higher than the axis of the bore, and the part of the curve near the muzzle is very gradual. Because of this, you will notice that after 25m the curve is actually above the line for a while, until the curve crosses back over the line out at around 340m. What this means on the target is that if you keep that adjustment, then for that entire range between 25 and 340m you will be hitting above the spot you are aiming at. In fact out at 50m you will be 4" high, at 100m you will be 6" high, at 200m you will be 9" high, and at 300m you will be 4.5" high. If it makes you feel better, in combat you would probably still be successful if you aim center mass on man size targets at these ranges without adjusting your sights. Since you probably spend more time shooting paper and small targets at under 300m I don't think you want a 25m zero with the rear sight bottomed out. For the good old military issue rifles with standard issue ammunition, the markings on the A2 sight elevation knob actually correspond to distances. 1/4 means 100m and 400m. 5 means 500m, 6 means 600m. For ranges under 100m, the error will be less than the distance between the sight and the bore axis, which in military terms is too small to worry about. You might find a small z marked on the knob somewhere, this would be where you should zero IF your barrel length, twist rate and ammunition matches the rifle that the sight knob was designed for. If your specs aren't the same (or if like most commercial rifles you have no clue what spec the parts a built to) it probably is still close enough to make a good starting point. Or you could get out some ballistics tables and figure out exactly what is the highest point in the trajectory, then shift all your settings so that the rear sight bottoms out at that point. This would give you the maximum available adjustment for long distances. Or you could do like most AR owners and just shoot it and be happy to make holes in stuff. |
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I'm some form of savvy to the science behind ballistics; bullet travel. Gravity pulling a bullet towards earth, thus you must adjust your barrel axis to compensate for. The idea is that at 25m the bullet hits point A and drops into point B which is generally 300m according to military manuals. There is some controversy depending upon the mathematical accuracy behind that formula. There are a lot of variables involved as well.
I guess I've made a big stink over what should have been a basic question. Where should I set my rear aperture for my 25m zero, with a range of 107 clicks, if my adjustment has no markings on it? |
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Read through this:
FM 3-22.9: https://rdl.train.army.mil/catalog/view/100.ATSC/434948EB-8945-4C25-9389-B33B4927856C-1308726234899/3-22.9/toc.htm#toc or if you prefer PDF format: http://www.benning.army.mil/infantry/199th/DCO/content/PDF/FM3-22-9.pdf It should get you introduced to some of the basic concepts. -See Chapter 5, Section II for zeroing procedures. -See figure 5-25 for an illustration of where rounds will impact relative to the front sight post at various ranges for a 25m-300m zero. -Chapter 2, Section V has a bunch of bullet trajectory diagrams, and elsewhere in the FM there are various ballistics charts. |
| I've read through a few field manuals. They have good information, but they do not answer my question. The military stuff refers to a rear aperture adjustment marked 8/3 or 6/3..I have neither. I have a knob with no markings on it. There lies the issue. Where should I keep my rear aperture elevation wheel set when zeroing my front sight elevation? (keep in mind, no markings on my rear elevation wheel; I am looking for a number of clicks maybe? my rear is 1/4 by 1/4 with 107 clicks from top to bottom) |
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You'd like to set your front sight height so that your rear sight is a few clicks above bottoming out at the peak of the trajectory (100y-ish) . This allows you to dial a 100y zero if you need it. Now your front sight post elevation is set and hopefully you'll never touch it again. While getting this initial zero, you'll also get your windage dialed.
Then figure out how many clicks off the bottom you have for a 50/200y zero, a 25/300y zero, 400y, 500y+, etc. If your rifle is canted, your windage will be inconsistent at longer ranges, so watch that. When doing a 25/300y or 50/200y zero, always set and check that zero at the longest range in the pair (200 or 300y). You can't trust a 50y zero to get you very close at 200y, but a 200y zero is trustworthy at 50. Even more so for 25/300. |
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On my ballistic calculator with 20" barrel shooting M855, the top of the trajectory is close to 100m. This would be the spot where you are most worried about bottoming out. I personally would put in about 2" of cushion which at 100m is about 8 clicks. Too much cushion and you are selling yourself short for long range.
The whole procedure would be most simple if you zero at 100m with the rear sight 8 clicks from the bottom. Of course with some added calculation you can zero at any distance you want. From my trajectory table, you would need about 5.2 MOA to switch between 25 and 100m. So to zero at 25m, dial in your 8 clicks, then put in additional (5.2x4=20.8) 21 clicks. Just a heads up, you front sight adjustment is not going to be in units of 1/4 MOA. Get as close as you can with the front sight, then adjust the rear to fine tune. Now there is no controversy about the flight path a bullet takes, it's just that the calculation involves so many variables that nobody can do it in the field. The controversy is over what simplifications give an acceptable answer, and what simplifications leave out too much. And the answer to that depends on your standard of accuracy. Do you want to hit within the 10 ring, or do you want to hit a man sized target under stress? About zeroing at ranges longer than 100m, you must be careful to be realistic about what you will accomplish. Whether you measure 1/16" changes on your 25m target or 1/4" changes on your 100m target, the measurement uncertainty is about the same. If when getting your long range zero you fail to correct for the effects of atmospheric conditions, then you have just added a huge level of uncertainty to all your other shots. At least with a close zero, you have textbook theory to base future corrections off of. Use long range testing to verify that the theory is correct and/or record data that you will use to improve on that theory. |
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