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Sunday, August 30, 2009
Snipers with big bores are compensating for something. Guess what it is? If you double the caliber, and the bullet is the same shape, you roughly double the BC. So larger calibers can have more streamlined bullets. To the left we have a 22 vs a 338 for drop. The 22-250 is closer to line of sight to begin with, because it is faster. The 338 catches up at about 450 yards and has less drop after that. So what? Well let's look at wind. Streamlined bullets (Big BC values) are much less affected by wind. Here we have 3 times the wind drift at 500 yards. About 18 inches of a difference. That's why snipers prefer 338 Lapua to the smaller calibers. They aren't worried about the Lapua going 1 inch higher at 250 yards. They are worried about a sudden 10mph wind blowing them right off target outside 400 yards. They are spot on for elevation with a laser rangefinder and a note of elevation clicks for different ranges. The only way to miss is to misread the wind. And larger calibers have the big BC values. Mind you the increased recoil of the big bullets might require a bigger shooter. So, you guessed right! Technical stuff: D is diameter, M is mass. Doubling diameter cubes the mass if the shape is the same. So if the shape stays the same, BC is roughly a measure of length.
You may have been in the position where your shots are going 4" high and left at 200 yards and you want to adjust your scope. You can move the scope a few clicks and see, but ammo is expensive and we don't want the stockpile to get low. Most scopes have 1/4 MOA clicks. One MOA is 1" at 100 yards*. So a click is a quarter inch at 100 yards.
Range/Yards Click Size, 1/4 MOA 100 1 quarter inch 200 2 quarter inch (half an inch) 300 3 quarter inch 400 4 quarter inch (one inch) 500 5 quarter inch (one and a quarter) (50 1 eighth inch)Some scopes have 1/8 MOA per click for finer adjustment:
Range/Yards Click Size, 1/8 MOA 100 1 eighth inch 200 2 eighth inch (quarter an inch) 300 3 eighth inch 400 4 eighth inch (half inch) 500 5 eighth inch (50 1 sixteenth inch)OK, you can remember that. So 4" high and left at 200 yards again. 200 yards, 2/4" per click. Half an inch per click. So 8 clicks=4" on the usual 1/4 MOA scope. Which way to turn the scope knobs? One knob has an "Up" arrow. Turn in that direction to move the bullet impact up. One knob has an "Right" arrow. Turn in that direction to move the bullet impact right. The markings on the scope are talking about bullet impact. So if you are shooting high, move in the opposite direction of the "Up" arrow. Two things to remember:
- A .25 MOA click is 1 quarter inch at 100 yards, 2 quarters at 200, and so on.
- The markings on the scope are talking about bullet impact.