Thread: 1000yrd load???
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Old January 31, 2013, 11:21 AM   #16
Join Date: March 4, 2005
Location: Ohio
Posts: 13,648

You reminded me that the throating reamer I have for .45 Auto also stretches the leade angle out. My Goldcup's factory barrel, once fit by the old weld-up and scraping method and throated with that reamer, shot very well. This, despite the bore being half a thousandth out of round at the muzzle.

Also, Harold Vaughn made his own chamber reamers with just 0.75° throats, which he felt helped with accuracy.

This appears to be an example of the hole and cone principle, complicated by the softness of the bullet jacket relative to the barrel. Using a conical pin to force centering in a hole is an old geometric principle for locating and indexing things, like the tool head turrets on lathes, but they use hardened pins and hardened conical inserts for that. When you look at a soft bullet hitting a throat with a steep leade angle, you can just imagine the contact is too abrupt to steer the bullet to center before deformation occurs. A more gentle steering slope will meet with less inertial resistance. So it seems, for the secant ogives, that the ogive's angle of departure from the bearing surface may be too steep, like using a steeper throat angle, except it's on the bullet. That may be why the hybrid shape is better.

Walt Berger also opined that the reason moly-coated bullets would often exhibit slightly higher ballistic coefficients at long range than their uncoated counterparts had to do with the lubricant helping the bullet center before deforming. That BC gain, typically just a digit or two in the third decimal place, was never really been enough to matter on paper, but it can be measured. The idea is that a bullet that enters the bore more straight exits more straight and thus has lower initial yaw and attendant slightly lower drag in the first hundred yards or two while coning settles into the yaw of repose.

What interests me in Berger's seating depth testing is their finding seating depths with VLD bullets set back as much as 0.15" off the lands that get better accuracy than when touching the lands. It matches the Somchem experience in item 3. of this old page. There may be a number of explanations for this. I have some experiments in mind for this with throat design. I'm going to start with a .30-06 bolt gun on seating depth and maybe hit it with an experimental throating reamer later to check the effect. But the reamer has to be designed and made so it will be some long term exploration.


OK. I'll be more careful in the future. I'm down to my last two slips of those things, and not the same size. Scary.
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