What happens with the shorter bullet is it has a higher gyroscopic stability factor than a longer one does when fired at the same velocity and with the same rifling pitch. That causes it to settle out yaw in fewer coning cycles. But because it's shorter, it is also has more gyroscopic rigidity so that each coning cycle takes a greater distance to complete. Within normal ranges of gyroscopic stability factors used in shooting, that tends to neutralize the advantage of the fewer cycles required to damp out yaw. Net stabilization distance difference: often not significant.
You can look at 6 DOF program results on damping of yaw for different bullets at different stability factors in Harold Vaughn's book, Rifle Accuracy Facts, and in Brian Litz's book, Applied Ballistics for Long Range Shooting. Assuming you aren't talking about a bullet whose stability factor is barely over 1.0 and that is almost unable to ever stop coning, they all seem to take about 200 yards to damp about 80% of the yaw out. The yaw is not great, so they may appear to be fine at 100 yards. It depends how small a group you are shooting.
Litz confirms that flat bases are usually easier to get tight groups with at shorter ranges, but bear in mind this is tight the way a benchrest shooter sees it. If you are interested in position shooting, chances are you won't see a difference unless your muzzle crown is off. Litz puts the reason to what JD0x0 mentioned: It's relatively simple to make a square base symmetrical, just like putting a 90° crown on a rifle is easy to get right. But as you create an angle, getting perfect symmetry where the conical angle meets a straight cylindrical surface is more difficult. The steeper the angle a boattail has that meets the bearing surface, or that a crown has that meets the bore, the greater the longitudinal error the same amount of offset creates. It's proportional to the cotangent of the included angle or twice the cotangent of the half angle. A 0.0001" offset of the cone axis of a 9° boattail will produce 0.0013" of longitudinal unevenness where it meets the bearing surface.
I will add that a boattail takes longer to finish clearing the edge of the muzzle crown than a flat base does, so the boattail dwells in the highest speed region of the muzzle blast jet longer than a flat base does as it exits the bore. This means any small imperfection in the barrel crown or any unevenness in the distribution of unburned particles moving forward in the bore and deflecting off the bullet base will tend to have their influences on initial yaw and any lateral drift component they impart exaggerated by that extra dwell. The boattail should, therefore, do best with a load that burns up completely in the bore, produces the least muzzle pressure, and is fired from a barrel with a perfect crown. A long barrel with a target crown looks good from these standpoints.
Another muzzle blast factor is that blast deflects off a square bullet base with good perpendicularity, translating momentum mainly forward. But with a boattail it also deflects off the tapered sides, tending to push inward on the bullet base. If there is any imperfection of coaxially of the boattail with the bearing surface of the bullet, this results in net greater push on the longer side, pushing the bullet to the side and instilling a lateral drift component that stays with the bullet all the way to the target. I have actually seen this unevenness in vendor photos of some cheap import BT "match" and FMJ bullets. If you desire a bullet with the accuracy of military ball ammo, this is one way to get it.
All that said, the modern quality name brand match bullets all seem to be awfully good these days. I'll repeat that it's hard for anyone not shooting consistent cloverleafs to really see a difference even off the bench. Getting the right stability factor seems to matter more.
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Last edited by Unclenick; October 14, 2013 at 03:04 PM.
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