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Old March 30, 2013, 04:45 PM   #60
Unclenick
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Join Date: March 4, 2005
Location: Ohio
Posts: 10,247
Quote:
Originally Posted by Scottish Highlander
…With the 308 I used to use 125 grain shop bought but I have now moved up to 150 grain reloads to try longer shots. On a normal day stalking your dealing with wind in Scotland so the heavier round helps.
Be aware that as far as wind goes, it's only velocity and ballistic coefficient that matters, as ballistic coefficient already takes weight into account. So you can have a smaller diameter and lighter bullet match or beat the wind drift of a larger diameter and heavier bullet if its ballistic coefficient is equal or higher and if the velocity is equal or higher.

All the ballistic coefficient does is scale the aerodynamic ability to retain velocity and resist wind to that of a 1 inch diameter, 1 lb reference projectile at the same velocity. The idea is that if a bullet has a BC of .500, it's half as good at retaining velocity against drag and at bucking wind as the reference projectile is at the same velocity. If the BC is .333, then it's 1/3 as good, etc. So as long as both bullets are compared to the same size and shape reference projectile and at the same velocity, then the factors of shape and weight are already allowed for.

The only place the system comes apart is when the reference projectile shape is different from the actual bullet shape, then the BC will change with velocity because the different shapes have different drag coefficients at different velocities. This is usually the case, as the number you are usually given by bullet makers is for the G1 shape reference projectile BC at or near a typical muzzle velocity, only. That's actually not a very close match to modern bullet shapes, so you find Sierra, for example, giving you different BC's at different velocities, and Berger giving you G7 BC's in addition to the standard G1's, so you have a better matching shape to compare to, allowing one BC to work across the range of velocities pretty well.

In your .308, if you launch a 125 grain flat base bullet with a BC of .28 at 3000 fps, at 300 yards a 30 mph wind will move it 33 inches. A shot perfectly centered in the sights will move 5 inches and be outside a 10" circle a 123 yards. But if you launch a 180 grain boattail with a BC of .5 at 2600 fps, at 300 yards it will be off 21 inches, and won't leave a centered 10" circle until 150 yards.

Anyway, you can see that you want to look not only at the chambering but at what velocities you can get and what bullet BC's are available that will stabilize in your gun's barrel twist rate.
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Last edited by Unclenick; March 30, 2013 at 04:50 PM.
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