Re: What the heck is goin on?
At those close ranges the bullet may still not be fully stable and that will cuase a low BC or high initial velocity loss. The 175 SMk is a the upper range of what you can expect to stablize. I assume you have a 9.25 twist barrel. After a couple of hunded yards you may regain the high BC from this bullet. Be careful not to shoot you chrony if you try to measure it out there. I recently tried the 175 SMK but the 160 AB with a long bearing surface and not so much boattail stabilizes better for me.
here is what Sierra says and I expect that it applies to your case because it is a match type bullet you are shooting
. Any of the firing test methods measures a ballistic coefficient of the bullet as it flies through the air, including effects imparted by the gun, the cartridge, and firing point environmental conditions, as well as imperfections in the bullet. Theoretically, the BC of a bullet depends only on its weight, caliber and shape. But in a practical sense, the measured BC of a bullet also depends on many other effects.
The gun can affect the measured BC value in two important ways: spin stabilization and tipoff moments. A bullet is gyroscopically stabilized by its spin, which is imparted by the rifling in the barrel. If a bullet is perfectly stabilized by its spin, then its longitudinal axis (which is also its spin axis) is almost perfectly aligned with its velocity vector. If a bullet is not perfectly stabilized (which is usually the case), the bullet may not be tumbling, but its point undergoes a precessional rotation as it flies. In previous editions of Sierra’s Reloading Manuals we have described this precessional rotation and have called it “coning” motion to aid in mental visualization of the motion. As the bullet flies, the point rotates in a circular arc around the direction of the velocity vector. <font color="blue"> Coning motion results in increased drag on the bullet, and any firing test method then yields an effective BC value for the bullet that is lower than the theoretical value.</font> The rifling twist rate in the gun barrel and the muzzle velocity together control the spin rate of the bullet, and therefore control its degree of stability.
When a bullet exits the barrel, it generally has a small angular misalignment, which ballisticians call “yaw.” Yaw is caused by tipoff moments of torque applied to the bullet by powder gases exiting the barrel nonsymmetrically around the bullet, or by barrel whip or vibrations. This angular misalignment will cause coning as the bullet begins to fly downrange. Coning can also be caused by an abrupt exit of the bullet from the barrel into a crosswind, although BC measurements should never be attempted when winds exist at the firing point.
The cartridge used in the firing tests affects the measured BC values mainly through the muzzle velocity it produces. As noted above, <font color="red"> muzzle velocity combines with the twist rate in the rifling to produce the bullet spin rate, which in turn controls stability</font>. In addition, BC values change with the instantaneous velocity of the bullet, and so the muzzle velocity directly affects the measured BC value of the bullet. For example, a 180 grain 30 caliber bullet can be fired at a much higher muzzle velocity in the 300 Winchester Magnum than in a 308 Winchester cartridge. The same is true for a 240 grain 44 caliber bullet from a 44 Magnum compared to a 44 Special. So, the measured BC values can be expected to be different just because of the different starting velocities.
The Smokin Fur Rifle Club