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Old March 31, 2013, 04:11 PM   #79
Unclenick
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Join Date: March 4, 2005
Location: Ohio
Posts: 10,222
Quote:
Originally Posted by Scottish Highlander
…at least 100 grains AND have a minimum muzzle velocity of 2,450 feet per second AND a minimum muzzle energy of 1,750 foot pounds.
That knocks all the normal .22's out, as 90 grain match bullets are about as heavy as are commonly available in .224" bullets, and even those need a 7" twist barrel to stabilize and they don't expand reliably. 100 grains is not uncommon in 6 mm (.243 in U.S. bullets), but I'm not seeing BC's as high in the 6 mm or in the .257's as you get with the 6.5 mm to 7 mm bullets. You could do a lot worse than the 140 grain Hornady SST in 6.5 mm, using something that pushes it fast to make it flatter shooting. But also good an BC are their 150 grain .270 SST or their 162 grain 7 mm SST. These have G1 BC's of 0.520, 0.525, and 0.550, respectively.

One other factor to consider is how much flat shooting really matters to you, and that depends in part on how you sight in your rifle. Let's take your .308 Winchester and the 6.5-284 I mentioned earlier. Say that we load the Hornady 180 grain Interbond (G1 BC of .480) at 2600 fps in the .308 and the Hornady 140 Grain SST in the 6.5-284 at 2900 fps. I use Jeff Cooper's idea that you set the sights of any high power rifle to be on at 200 yards. That puts the bullet around 2"-2.5" high at 100 yards for most cartridges and it then stays within a 5" circle from the firing point all the way to somewhere between 200 and 300. If you have a 10" aiming circle (deer), that leaves half the circle for the gun and half for the shooter's hold error¹.

So, let's see what the differences look like:

The .308, 180 grain Interbond, BC=0.480", 2600 fps MV, sight zero at 200 yards

Apogee 2.5" inches high at 111 yards
PB limit: -2.5" low at 235 yards
Correction at 300 yards: Hold 9.4" high
30 mph side wind drift at 300 yards, 22.2 inches

The 6.5-284, 140 grain SST, BC=0.520, 2900 fps MV, sight zero at 200 yards

Apogee 1.9" at 114 yards
PB limit: -2.5" at 244 yards
Correction at 300 yards: Hold 7.2" high
30 mph side wind drift at 300 yard, 17.3"

So, for all that flatter shooting, I gain 9 yards of point blank range and 2.2" lower holdover at 300 yards when using a shared 200 yard zero. It is the wind where the biggest difference turns up.

You could squeeze a little more out of the 6.5-284 by taking the zero range out far enough to get a 2.5" apogee, like the .308 had. This means sighting the faster round at 224 yards instead of at 200 yards. You get:

Apogee 2.5" at 125 yards
PB limit: -2.5" at 264 yards
Correction at 300 yards: Hold 5.6" high
30 mph side wind drift at 300 yard, 17.3"

So that gets you about 1.5 moa less holdover at 300. Anyway, I just wanted to be sure you got the idea that with practical sighting the differences aren't quite so big as the advertising hype might lead you to believe.
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