wayne,
hi, well here goes, I have 2 rifles a 45-70 highwall dave h chambered it, and a 40-65 ballard that i did, that have some "freebore" i have found that my bullets with the first driving band being groove dia are just shy of the rifling I had paul j modify his molds so the very first driving band is actually bore dia plus .001. and it is actually seated into the rifling with the second band now groove dia just shgy of the rifling thereby sort of gettiong a breech seating effect, and it works well.
when you hit a chunk of lead  with 15tons of pressure in the ass. it iss going to try to fill what ever space it has to go to.
hope this helps some
martin

Wayne:

One of the keys to accuracy is have the bullet exit the muzzle with its center of mass and center of form as close to the exact same axis as possible. In the bore it will rotate on its center of form. Out in the air it will transition to rotation on its center of mass, complicated by a bunch of other stuff I don't pretend to understand. The smaller this transition is the better.

The only way to achieve this oneness with the universe is to take a perfect bullet and align it perfectly with the axis of the bore, push it down the bore to exit spinning perfectly on its center of mass. No transition, no variables, world peace and the end of global warming.

The idea of a freebore aiding accuracy is that if perfectly cut true to the axis of the bore, the freebore can aid in the alignment of the bullet with the bore.

Benchrest shooters are using tight necked chambers and turned neck brass with zero clearance to align their jacketed bullets with the bore and, obviously, having very good success in doing so. However, for some reason that just does not seem to work real well with cast bullets. I could only speculate why that is, but it is so in my experience and in others.

I have settled upon a chamber neck diameter that is .003"-.004" greater than the diameter of the case neck with a properly sized bullet pressed in it. This is pretty tight compared to most commercial chambers, but downright sloppy by BR standards. It leaves the fired brass about .0015- .0025"  larger than the bullet. I do not use neck tension, but instead use the brass as fired and allow the chamber throat to align the bullet with the bore.

If used, the freebore is cut as perfectly in line with the bore as possible at a diameter .002" greater than groove diameter. The bullet used is sized to .001" larger than groove diameter.  In the case of a .45 caliber, we have a .459" bullet in a .460 freebore. This fit aligns the bullet with the bore considerably closer than is possible to obtain with case neck fit.

The bullet (and brass) does expand to fill all the available space when the big light goes on, so it is still important to have the chamber neck concentric with the bore and consistent neck thickness on the brass. Locating the front of the bullet with the throat and allowing the rear of the bullet to bump up into a bore concentric neck space is the best way to achieve consistent accuracy with fixed ammo that I have found.



Glenn Fewless

Glenn,

I read your comments again this morning and I believe it finally clicked.  You said, “I do not use neck tension, but instead use the brass as fired and allow the chamber throat to align the bullet with the bore.”  So to put it in layman terms, since you do not use neck tension, the bullet is not restricted from moving slightly in the case neck.  Since the bullet diameter is held to only 0.001” in diameter less than the freebore diameter, the freebore performs the final alignment.  Is my understanding correct?  If so than my hang up was with the terminology.

Per SAAMI definitions, leade and throat are one in the same, and does not include any freebore.  Therefore, when you said you “……..allow the chamber throat to align the bullet……” I initially read this as being the same as using the leade to aid in aligning the nose of the bullet, a technique many shooters use with or without freebore.

Wayne