Been looking at a barrel for my next build and I'm like the feed back I've seen from the members here who have bought Criterion Barrels from Southland Shooting Supply.
They list a number of barrels as having "Match" chambers. I'm very interested in another 6.5-06, but the chambering list, lists it as a Match chamber.
Not a lot of 6.5-06 brass out there, so I will go down the once fired road...AGAIN! And I hope not to hear the same BS I got from another barrel maker..."if you use once fired brass, you should expect to have problems!"
So what is a match chamber compared to a "normal" chamber?
I'm my experience it mean many things. Many of us build a dummy round with a specific bullet we intend to shoot and have a reamer made to that exact round. We spec jump, od, neck and the oa length and Clearances. As in .258 neck. .0025 OD and .0015 headspace/shoulder clearance. Not in those exact terms but to accomplish that end.
Most SAAMI Chambers are reamed at the max saami spec. Most factory brass is made to fit the min saami spec. Most FL size dies are made to resize fired brass to the min saami spec. This is great for hunting and to ensure that all ammo manufacturers products will fit in all firearms manufacturer's guns. This is not however a recipe for the most accurate rifle/ammo combo.
If you ever measured new brass and once fired brass after firing it in a saami chamber, the new brass can be .005"-.008" shorter and smaller in OD than it is once its fired. This is why many of us fire form our brass and buy neck only size dies and bump dies to set the headspace/shoulder clearence to .001"-.002". The trick is to have the brass match the chamber so well that it comes out almost the same as it went in. (No growth)
When you fire the gun the case exapands. It matches the chamber in every dimension. As it does, it physically locks up against the chamber walls reducing stress on the bolt lugs. When the pressure drops the case rebounds (shrinks) a couple thousandths.
When you then full lenght size the brass it returns the brass to its "new min saami spec unfired dimensions" which means that you reduce the size .004" to .008" It will then grow .004" in most dimensions or more with every firing. This excessive stretching and forced resizing causes too much case growth in firing and can shortens case life thru a process called work-hardening. If you bend metal back and forth it will eventually break. Every time you bend metal it loses its elasticity. It gets harder and harder. (brittle) The more a case is resized after firing the more work hardened it becomes and the less consistent neck tension will be. This will effect pressure, accuracy and increase velocity variations. Unless you anneal your brass it will crack and often seperate.
In a perfecf world the idea is to have a chamber that would be perhaps .002" over the min saami spec in both lenght and OD at the datum points. So I might buy lapua brass. Use the FL die I intend to use to full lenght size it. Then make a dummy round with the bullet I intend to shoot and send that round out to have a chamber reamer made to give me a couple thousandths clearance at all datum points. In effect I have a custom chamber and all my factory dies are now basically custom match dies that size to that custom chamber.
The chamber still falls within the overall SAAMI tolerances. It's just a tighter spec matching the chamber and dies. This is a recipe to get the most accuracy from a rifle. It is not a recipe for a hunting rifle as any dirt or even a fouled chamber can result in a jam.
I like Manson's standard reamers for hunting rifles. I've built some rifles with his stock reamers that shot 1/8" @100. He spilts the saami spec. I have never had a jam on any rifle and they all shoot providing your smith does his job.
The last part of a match chamber is the process. If match procedures and care are not used in the set up, turning, threading, chambering and assembly then it really doesn't matter what reamer you think you are getting cause you ain't.
