Wednesday, December 16, 2009

The Thousand Yard Conspiracy - Sidebar: Why not .30-06 for long range?

Monday, as part of my 1000 yard conspiracy post on lower cost long range shooting, I went into a little detail on selecting a chambering for your precision rifle.

For that discussion, I suggested .308 win, and .300 winmag; because of accuracy, power, ammo availability, and cost.

There is another option in the .30 caliber family that can fire the same bullets as the .308, but a bit faster: the .30-06.

Since it has a bit more powder capacity, and can push the same weight of bullet to higher velocities (or a heavier bullet to the same velocity); you might think it would make a better choice for long range shooting than .308, but most long range shooters choose some other chambering.

Several people have asked why that is.

Firstly, I just want to say I'm a big fan of the .30-06 cartridge as a whole. I own two .30-06 rifles. It's good for most north American game at under 400 yards (at least with proper load selection). It's low cost (relatively speaking... ammo aint cheap these days).

Plus, the '06 is a great American tradition. There are probably more American bolt action rifles chambered in it than any other centerfire caliber. Carlos Hathcock made almost all his kills with a Winchester model 70 in .30-06.

In fact, the '06 has killed more badguys for us than any other chambering. That title will probably last until we get man portable battlefield nuclear sidearms, given the mass casualties of WW1 and WW2.

...It's just not quite as accurate as other choices.

It's not bad at all, but it's enough of a difference, that most precision shooters will make another choice.

The issue, is one of consistency. For best precision, it is important to minimize the shot to shot deviations in pressure, initial velocity.. well really, every possible variation.

The .308 and .300winmag are slightly more consistent, because of the shape of their cases; and the shape of the powder column inside the case.

Basically, the case of the -30'06 is longer, but the same diameter, as the .308; giving the powder column a higher aspect ratio (or more technically, a low surface area to case volume ratio for the powder charge).

In cases where the powder column has a low surface area to volume ratio, the powder burns less consistently, and the high pressure gasses expand out of the the case less consistently; than from a lower aspect ratio case (a shorter, fatter, or both, case; which produces a higher surface area to volume).

This reduces accuracy to a small, but noticable, degree.

The wider diameter (and therefore more surface area of a powder charge) per volume of a case (to a point anyway), the more consistent the velocities and pressure achieved out of that case.

This improves accuracy to a small, but noticeable, degree.

This is why .308 and .300 winmag are more commonly chosen for long range accuracy than the .30-06. It is also why the .300wsm, and other similar, short, fat, cartridges are becoming so popular