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Posted: 10/8/2005 3:48:44 PM EDT
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Hi guys! I was wondering if anyone could help me here. The following is a set of performance tables for 168 Grain Federal GM Match Ammuntion .308 ammunition: http://www.snipercentral.com/308.htm I'm not 100% sure how to interpret the bullet drop data. Specifically the + signs. For example at 100 yards F-168 gr. says +17.7, 200 yards +31.0. Am I correct in assuming that the plus is referring to bullet climb while the - signs are referring to bullet dropage? So lets say at 100 yards the bullet will climb (because of the angle on the barrel) 17.7 inches from the muzzle? So out to 500 yards the bullet is still 23.5 inches above the muzzle. At 600 yards the bullet now falls level with the muzzle and at 700 yards it has now fallen to 36.7 inches below the muzzle? First question is, am I interpreting this correctly? If not how do I interpret this? My rifle is zeroed in to shoot dead on at 100 yards(1/2 - 3/4 inch groups). So at 200 yards my rifle would still shoot 13.3 inches above the bullseye and at 300 yards 19.5 inches above the bullseye? Somehow that don't seem exactly right but I am new to precision rifles. Can anyone please help me out here so I can calculate my scope settings? Thanks! |
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The information is correct USING A 600 YARD ZERO. If you ZERO at 600 yards with the stated ammo, then yes, you will be "high" by the #'s given (in inches) at the appropriate ranges stated. ETA: If you are zeroed at 100 yards, your bullet will only be @4" low at 200 yards, and @14.4" low at 300 yards. These are rough figures for a 168 grain HPBT-Match style bullet leaving your barrel at approximately 2700 FPS, which is just a tad over the factory Federal Match load if I remember correctly. Any good reloading book will give you the "Ballistic Coefficient" (BC) for a specific bullet. Basically, the BC of a bullet is a numerical figure that represents the bullets ability to overcome atmospheric resistance, or the "air resistance" as the bullet is in flight. The higher the BC, the more efficient the bullet travels through the air, thus, flatter trajectory. For example, a 150 Grain Round Nose bullet has a BC of @ 0.266 while the same weight bullet in a Boat Tail Spitzer design has a BC of @ 0.423. Know the BC of your bullet, the velocity in which it leaves your barrel, and any good Ballistic Coefficient table will give you data on the trajectory of the round in flight using various ZERO ranges. I have found the Speer BC tables to be quite accurate |
bullets do not climb they only drop.
17.7" above your line of sight/point of aim with a 600 yard zero.
depends on the bullet,velocity, and zero, usually 100 yard zeroes are low at farther ranges. On the page you linked there is a chart (4th one down) for GM 168s zeroed at 100 yards, point of impact at 200 yards is 4.5" below your line of sight and 15.9" below your LOS at 300 yards. I didn't confirm the numbers but they look close enough. You can calculate your trajectory here BC varies with velocity, Sierra lists them at their page. |
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Specter, Remember that these are also approximates based on the test rifle. Your rifle's actual performance will vary on barrel length, and scope height above the bore. If your rifle is zero'd at 100 yards, then try a ballistic table for a 100 yard zero. If you scroll down on the page you posted, you will see some 300 yard tables for the Federal 168 loading, where at 200 yards your 4.5 inches low, at 300, you are 16" low. It gets to be fun when you get yardages like 732yds, and if you have a ballistic chart of your rifle's actual performance taped to your stock, you can dial in pretty fast, but the farther away you have more drop to battle. |
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Ok I have another question. I downloaded Federal Ballistics calculator which is awesome! It will factor in temperature, elevation, windage etc. starting at any zero you selected. Anyways I have one more question before I can do these calculations. My scope (Leupold VX-2 Tactical) is 1/4 MOA adjustments. I know that that this means that at 100 yards (which is where I zeroed in my rifle), 1 click will move the bullet 1/4 inch. My question is that at 200 yards how much will one click move my bullet? Does anyone have any reference tables that I can look at on this? I want to reference my scope settings out to 800 yards in 50 yard incriments. Also does anyone have a Leupold VX-2 or 3 tactical scope? I lost my users manuel and I have a question. I zeroed my rifle in at 100 yards and the settings were as follows 7.1 elecvation, 8.1 windage. Now if I loosen the hex (set screws) on the rim of the knob until the dial turns freely I can reset my zero number to say 0 without messing up my adjustments correct? Thanks guys I appreciate the feedback. |
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sounds like you need more than tutoring if MOA is a mystery riflestocks.tripod.com/moa.html 1MOA is ~1" at 100 yds or 2" at 200, 1/2MOA is 1/2" at 100 or 1" at 200, or 1.5" at 300 etc, bullet goes twice as far it's twice as wide from the POA. 1/4MOA is 1/2" at 200. the hex screws allow you to set the knob to the range you zeroed at after zeroing. might be an owners manual here |
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Think of any firearm as minature artillery. "Point Blank" is the distance from the muzzle to a point of impact without any noticable bullet drop. The range at which you zero your scope could be before or past the apogee - it is possible to have 2 ranges that will both be zeroed. I know it sounds silly but if you graph it out it makes alot more sense. ( I think I have totally confused the issue, if not add more smoke.) |
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The key to understand external ballistics is to look at a graph and realize a bullet will cross the line of sight twice. Once on the way up since the bullet starts below the line of sight, usually by around 1.5 inches or so that the center of the scope or sights is over the bore. This is called the near zero. The bullet continues to climb until gravity overcomes the upward arc. The bullet then falls to earth over a suprising long distance. When it crosses the line of sight for the second time you have the far zero. With the .308 or .223, a near zero of around 25 yards will produce a far zero of 200 yards. Good luck with a journey which never ends. |
IF the rifle is zeroed that way. It is quite possible to have only a near zero and no far zero. Bullets do not climb - they fall from the moment they leave the muzzle, gravity affects them the entire time with equal effect. |
| I'm fairly new to trying to reliably hit the target beyond 300 yards, and found the book "Dead On- The Long Range Marksman's Guide To Extreme Accuracy" by Noblitt and Gabrilska to be a good introduction to the subject. In the field I found there was definitely a difference between what I could locate in published tables/ on-line ballistics calculators and what my rifle/scope/ammo combination would actually do. By using vertically doubled paper targets at longer ranges I was able to hit paper and determine if my "come-ups" were about right, too much, or not enough. Still working on it, and it is a fun/challenging way to spend a Saturday afternoon. |
The near zero is the point the bullet crosses the line of sight. Trust me if there is a near zero you will have a far zero. The bullet actually does climb compared to the line of sight since it starts below the line of sight and must be aimed up assuming of course that you want to hit something. You are correct in stating that gravity is affecting the bullet from the moment it leaves the bore which is why the far zero is where the bullet passes through the line of sight for the second time on its way to the ground a few thousand yards down range. What is confusing you is the line of sight which is a straight line to the horizon and the path of the bullet which starts below the line of sight, climbs through the line of sight on the way up and then falls through the line of sight on the way down while falling to earth. When you have this all figured out and the affect of sectional density and ballestic coeficient of the bullet, elevation, barmoetric pressure, temperature and other factors, we can move on to correction for wind which is way more difficult. It's a life time of learning. Good luck. |
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Ok as i'm calculating this I want to make sure that I am correct on this. Whether it is elevation or windage MOA would increase as follows: 1/4 MOA scope adjustments = 1/4 inch at 100 yards, 1/2 inch at 200 yards, 3/4 inch at 300 yards, 1 inch 400 yards, 1.25 inches at 500 yards, 1.5 inches at 600 yards, 1.75 inches at 700 yards, 2 inches 800 yards? |
In theory, yes, based on geometry. |
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The math is actually pretty simple. 1 MOA is 1/21,600th of circle's circumference (i.e. there's 21,600 minutes per circle: 60 minutes/degree, 360 degrees in the circle). To calculate the linear distance per MOA for a given range, you just take your range, multiply it by pi, and divide it by 21,600. Watch your units, to go from yards to inches you'd need to take your yard MOA and divide it by 36 (3 feet per yard, 12 inches per foot). So, our 100 yard range has an actual 1 MOA value of 1.047198 inches. 100 meters would be 1.145229 inches (or 2.908882 cm). For other distances you can just multiply and divide by the ratio to 100 yard/meter range. 75 yards = 3/4 of 100. 623 yards = 6.23 times the 100 yard value. And you were wondering when that 8th grade geometry class would come in handy 
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