onondaga
posted this
06 August 2013
http://www.castbulletassoc.org/view_user.php?id=2607>biddulph
Chamber throat begins where the area in your chamber for the case neck ends and the throat continues to where the rifling begins and tapers to full dimension of the lands and grooves. Throats are usually parallel walled with constant diameter from beginning to end but some are tapered and that would be measurable from the chamber casting of your rifle by using a chamber casting alloy cast into a chamber. The term “Ball Seat” is a military term used to define the spot in a tapering throat where the diameter of the bullet and the throat match and are equal.
The most accurate way to measure chamber throats is with a special chamber casting alloy poured into a chamber that has been prepared with a temporary plug of rag precisely located.
You can get an inexpensive chamber casting alloy From RotoMetals and their instructions are pretty good. you can also search videos on “Chamber Casting” through Google or YouTube, there is a bunch of them.
RotoMetals chamber casting alloy:
http://www.rotometals.com/product-p/chamber_casting_alloy.htm>http://www.rotometals.com/product-p/chambercastingalloy.htm
melting temp for this alloy is listed at 158-190 degrees F. so only low heat is needed and very convenient.
The alloy is re-usable, so one ingot is about a lifetime supply for one person.
For best accuracy with cast bullets, the bullet diameter should match the throat dimension with a very slight drag denoting a fit to the throat by the bullet. I have done this for all my rifles used with cast bullets to determine the best bullet size for accuracy. The best size is just that..the best fit to the throat with no wobble of the bullet possible in the throat when loaded or upon firing cast bullets.
This is what so many are saying ( myself included ) when they recommend that bullet fit is so important with cast bullets and recommend not to bother selecting bullet casting molds until you really know what bullet size you need, as throat size of your chamber defines the best fit for cast bullets. After that everything else is secondary.
Gary