Shooter's Bible Guide to Rifle Ballistics by Wayne van Zwoll

Author:Wayne van Zwoll
Language: eng
Format: epub
ISBN: 9781620872857
Publisher: Skyhorse Publishing
Published: 2011-02-28T05:00:00+00:00

There’s more to milk from standard deviation. With it, you can construct a bell curve that shows how your velocities grouped around the mean, and, for any given speed range, the percentage of shots likely to fall within that range. Occasionally you’ll get a velocity reading you don’t believe. It may be the chronograph didn’t register the bullet’s passage properly. Or the load was somehow defective. One rule of thumb to keep SD useful: Throw out any reading more than 2 ½ times the SD from the mean.

Bullet velocity varies not only with the powder type and charge and bullet weight; it’s influenced by chamber and barrel dimensions, throat shape and length and bore finish. A tight chamber reduces the amount of energy lost in case expansion. So does a tight throat. But a long throat that allows the bullet to move before engaging the rifling and permits long seating of the bullet to increase powder space enables a handloader to add fuel, boosting velocity. That long throat is generally thought to be less than desirable for accuracy; but on hunting rifles it’s not a liability. Roy Weatherby used long throats and ambitious Norma loadings to give his magnums lots of pep. My pal John Burns, one of the principals in GreyBull Precision, insists that long throats cut to a minimum diameter sacrifice no accuracy but give you more throttle than the short throats commonly associated with target rifles.

Bore dimensions and rifling type affect pressures and bullet speed. Some barrels seem to “shoot faster” than others that by all appearances should give the same performance. Bore finish is pretty much invisible, but you can feel it. The consensus of many barrel-makers is that smooth lands and grooves boost accuracy. “But you can get a bore too smooth,” one pointed out. “If you make it glass-smooth, you increase friction. It’s sticky, like the surface of a glass table.” Increased friction means higher pressures, which can increase speed; however, if your ammunition already tops the velocity charts, cork-popping pressure is not what you want. More velocity without higher pressure is the goal.

Barrel length also makes a difference in bullet speed. How much difference per inch depends on the original and finished lengths, the original velocity, the cartridge and powder and bullet. The only way to tell for sure how much velocity you gain with a long barrel or lose with a short barrel is to chop a very long barrel shorter an inch at a time. Such tests show great variation. One, conducted by A-Square with a .300 Winchester pressure barrel, measured velocities at 1 inch increments from 28 down to 16 inches. Loads of 70.5 grains IMR 4350 with a 150-grain Nosler Ballistic Tip, and 78.0 grains RL-22 with a 180-grain Sierra Spitzer gave these results:

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