Where does a cartridge sit in the chamber ?

[Bart B.]

As long as case neck wall thickness doesn't vary more than .001", I don't think they need to be turned or reamed for excellent accuracy.

A full length sizing die so modified with a larger neck diameter doesn't require any extra work on rimless bottleneck cases. But I think it's best to get a second die to size belted cases all the way to their belt reducing that tiny ridge a few thousandths in front of the belt that's left after firing cases sized in standard dies. www.larrywillis.com sells a collet die for belted cases that's well worth the money if best accuracy with them is an objective. I made my own years ago and use it after running fired belted cases through a standard full length sizing die whose neck is .002" smaller than a loaded round's neck.

Cases deprimed and cleaned before lubing and sizing in such a die ends up with a cleaner process. The use of a case headspace gauge such as the RCBS Precision Mic can help get the die set properly in the press so the fired case shoulder's not bumped back more than .002" for best accuracy and long case life. Dies with necks opened up don't need expander balls; cases last longer as their neck brass is less work hardened.

Standard dies have to work with all sorts of fired case dimensions as well as chamber and bullet dimensions. That's why they size necks way down then expand them to hold the smallest diameter bullets and make cases fit all sorts of chambers.

 

[Sierra280]

Wow, so much great info on this thread! (Thanks Bart B)

Since we are talking about how the cartridge sits in the chamber: Would indexing fireformed cartridges make any measurable difference in accuracy? I know the brass should fit the chamber fine anyway but because of concentricity or other variations would this be worth doing? (however, kind of impractical)

 

[Bart B.]

Sierra280, indexing rounds in the chamber so the bullet runout high point is always oriented the same way. Otherwise, I think it's a waste of time and effort. The bunting die that makes the headstamp on a new case isn't oriented to any physical property of the case. The case thick side can be at any point relative to the headstamp.

There is one other thing that can change how the back end of the case fits the chamber that changes from round to round when chamberd. Out of square bolt faces; common with all service rifles and factory sporter rifles. When a new case is fired in rifles with such a bolt face, the case head tends to flatten against it at peak pressure so the fired case head is not square with its long axis. Partial full length sizing that case then reloading it doesn't square the case head up. When its chambered again and the bolt closed on it with a slight crush fit, depending on how the high points of both case head and bolt face match up, that case may twist a bit and be forced against the chamber wall in some direction other than where it was when first fired. Rounds so chambered cause small differences in the direction the barrel whip axis is when fired; accuracy sufferes. All of which is a good reason to full length size bottleneck cases setting their shoulder back a thousandth or two so the bolt closes the same way on each chambered round. And having the bolt face squared up with the chamber axis if best accuracy is the objective.