Re: Holding over compared to dialing in MOA
First, have you made any angle indicators with the "correction factors" of
(1 - cosine), instead of the cosine or angle, or would you make one?
I've found that adding a flat 4 MOA to what ever my come-up MOA is for an angled shot, then multiplying it by the correction factor to see how many MOA to "take off" the level fire come-up MOA is way easier to remember the actual bore line drop this way. Saves another step in the field if I could look at the angle indicator and instantly see that the 35 degree correction factor is .181 instead of the .819 cosine on the cosine/angle indicator. Just one less math problem to solve, subtracting .819 from 1 to get the .181 I'm looking for.
Okay, I think that 1 inch being so close to 1 MOA both helps, and also confuses many people.
I'm not the best at explaining things, even things I understand well. I was confused by this when I got started figuring drops and corrections to dial, but it's pretty simple really.
I look at MOA and inches when I'm looking at compensating for a groups measured drop in inches on a target at a specific range, mainly. I really don't think in terms of inches, but rather MOA in most any other situation. In other words, I look at MOA on a chart, note everything in MOA, wind drift, on and on and on... Why think in terms of inches when you don't have to and just complicate things... It's like looking at things in terms of mils, but dialing in MOA, something I've never understood...
To me, if 1 MOA was equal to 1", that would really have people confused even more.
My turrets compensate for 1.047" (1 MOA) at 100 yards with every 4 clicks, or number on the turret. If I'm 8" low at 450 yards and want to know the correction to bring it up, first things first - Every 1 MOA I dial, the turret will move the crosshair 1.047" on the target image "at 100 yards", and if the target image is at 450 yards, 4.5 times farther, the crosshair moves 4.5 x 1.047" = 4.71". The crosshair moves lower when you "dial" for the bullet to impact "up". The crosshair moving lower forces you to raise the muzzle to bring the crosshair back up to the target, the more you dial, the more you raise the muzzle.
To calculate the correction for a specific bullet drop at a specific range, you must first understand that 100 yards is your base line, but not the multiple you'll be dealing with in the calculations. If the range is 678 yards away, 6.78 X 1.047 = the number of inches that 1 MOA dialed, or held over, will change the point of impact (POI). In this case, 1.047" at 100 yds (1 MOA) is worth 6.78 times times that, "at this range". Because it's 6.78 times as far, the 1.047" grows at the rate of 6.78 times also.
So, if you're 8" low at 450 yards, you divide the 8" by 4.50, then by 1.047 for the 1.70 MOA solution.
1.70 MOA = (8/4.50)/1.047
8/4.50 = 1.78
1.78/1.047 = 1.70 MOA
Another example -
If in the conditions you're shooting in, you know the bullet drops 109" at 815 yards from your 100 yard zero, (109/8.15)/1.047 = 12.77 MOA is what you dial to rezero for the shot.
Just remember, this is minute of angle not inches. When you dial up 1 MOA of elevation into your scope, you will be raising your bore line (BL) above your line of sight (LOS) 1.047" at 100 yards, but that BL is straight and extends to infinity at that angle relative to your LOS, and the divergance, or distance between this extended BL and LOS will be double at 200 yards (1.047" X 2.00), and triple at 300 yards (1.047" X 3.00) etc, etc... The distance between the LOS and BL, in inches, will be equal to the bullet drop from your 100 yards zero when this drop is correctly compensated for.
1 MOA equals 1.047" of divergance at 100 yards, range multiplies this as described. Divide the drop in inches at range by the range multiple, then by 1.047" to back into the MOA correction needed.
This load I've been working on and figuring angles with for a while now seems to be fairly consistant in one way that will speed up angle calculations some, or save me from having to carry a "drop from BL" MOA chart. This way I can work from my Come-Up MOA only, but retain the precision you only get from using BL drop in the calculations to find a corrected firing solution for angled shots.
My Come-Up MOA at whatever range is always 4.0 MOA less than my level fire BL drop MOA with this scope height, so adding that 4.0 MOA to my level fire range Come-Up, it yields the BL drop MOA I need in order to work the formula for a corrected come-up.
If you look at your BL drop in inches and convert it to MOA, then the same for your bullet path come-ups, subtracting the come-up MOA from the BL drop MOA at each 100 yard increment on out to 1500 yards, you'll see the difference between them at each interval the whole way out. With this 1.75" scope height of mine zeroed at 100 yards, mine runs an MOA difference of about 3.9 - 4.0 MOA all the way on out.
So, the formula to use with this now is, level fire come-up MOA + 4 MOA = BL drop MOA * (1 - cosine) for the correction MOA to go back and subtract from your level fire come-up, for a corrected come-up for that slope range.
Lased 900 yard shot, 45 degree angle:
Come-Up for Level fire 900 yds = 31.17 MOA
Corrected for angle come-up = 31.17 - ((31.17 + 4.0) * (1 - .707))
31.17 MOA + 4.0 MOA = 35.17 MOA BL drop
1 - .707 cosine 45 deg = .293 correction factor
35.17 MOA * .293 = 10.30 MOA correction
31.17 MOA - 10.30 MOA = 20.87 MOA corrected come-up for 900 yards, 45 degrees.
You can see how a "correction factor" angle indicator would save another small step in the calc's there.
Multiplying the 900 yard come-up (31.17 MOA)by the cosine (.707) gives a 22.03 MOA angle fire solution, and that's a 9.5" - 12" miss depending on which way you dialed using the other method, which is much more accurate.
If guys want to use holdover, the NF R2 reticle is at least calibrated in MOA, not mils, and that's what I'd use, and do. [img]images/icons/smile.gif[/img]
The Exbal program is nice, I use it as well, but it needs G5 and G7 drag curves to work at its fullest. It has some nice new features on the latest versions, the latest Palm version is really nice.
[ 02-04-2004: Message edited by: Brent ]