What is considered a match chamber and at what point do need to start turning necks down?

My tightneck 308 reamer is a .332". I used it on shortened 308 URBR cases called "30 Doggie" in HC. At this dia., it just "cleaned up" the necks.

As far as neck turning vs. chamber (inside neck diameter) dimensions is concerned, it depends on the brass you want to use. You'll need a minimum of about .002" clearance over loaded cartridge neck outside diameter to ensure safe release of the bullet. Much below that, and you'll have to start turning necks - either to reduce case neck wall thickness or simply to clean up necks for consistency. If you want to specify a chamber as tight as .002" > loaded neck diameter, you'll essentially have to settle on the brass you want to use (or want to be able to use - albeit with more clearance).

The last time I specified .308 reamer dimensions, I measured Lapua and Winchester brass. I believe I specified a reamer neck dimension of .340" to able to use Lapua brass safely in a no-turn configuration (I believe I came up with a loaded neck diameter of .338" for Lapua factory rounds). (PLease note: you'll have to establish all these measurements for yourself, don't take my word for what they might be).

In addition to the neck diameter, I specified the following reamer dimensions to ensure brass would function correctly, but without unduely having to work the brass:

Shoulder diamter: .451"
Web diameter: .471"
Freebore diameter: .310"
Leade Angle: 1.5 degrees

On top of that, I supplied a loaded dummy round to establish freebore.

Do the above dimensions make this a "match" chamber?? They probably distinguish it from a chamber to be used under the most adverse conditions in the field. Simply because a "field chamber" would benefit from more generous dimensions to work under dusty or dirty conditions. I'm not positive how much "minimal" chamber dimensions contribute to accuracy, although minimal chambers seem to be fairly common in competition type guns. Accuracy is probably a combination of many factors, one of which might be chamber dimensions, but other factors such as dimensional integrity may play as important a part as the chamber dimensions themselves.

- mike

I have a Obermyer Match reamer that I have cut at least four chambers with and no cases needed neck turning, including US Arsenal cases. A true "tight neck" reamer will be on the order of bench rest dimensions and much smaller (by comparison) than a "Match" reamer. You really need to do a "Cerrosafe) casting of your chamber to determine whether or not you will have to turn case necks.

quote:

Originally posted by CORNERSTONE:
What is considered a match chamber and at what point do need to start turning necks down?

A "match chamber" is someones derivation from standard and it's "match" because they said so. You need to start neck turning when the neck is larger than the hole you are trying to stick it into is. Jim