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I see where some have had their rifle chambered to shoot specific type of ammo such as the m118r.
1. Could someone explain to me what the details are as far as actual chamber dimensions?
2. And is this something done to rifles for actual combat use or just Match competitions?

Trying to figure if I want one of my 3 builds to lean toward this consideration or even if my perception is correct at all on this point.
 

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This has more to do with the finish reamer than anything. The difference is the shoulder and neck dimensions, the headspace can be done to any length. My chamber was finished with an Obermeyer reamer, there's also a .308 match reamer, the M118LR, M858 and a few others. The difference is hundredths to thousandths of an inch difference between all of them. Talk to a few guys and see what their results have been. So far I like the Obermeyer but finding places with it is tough. SEI is the only place I know of with the M118LR and M858 but I could be wrong. .308 match is the most common for commercial barrels.
 

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I don't know much about the raw technical details but the answer is yes some of us do chamber the barrels for a specific round. For example, when I found myself with five rifles to build and spent a year around hear soaking up everything I could I started buying tools. My first rifle was a SAI M21 and Inwas well stoked up on BH 175 gr match ammo. I don't know what SAI used to chamber the M21 I would assume it was a .308 match reamer. So considering I had all this 175 ammo and planned on using nothing but because all the rifles I built have Krieger match barrels and all the match mods, i started my finish reamer education. I was surprised to find out that there was a lot to know, more than I cared to learn actually. After speaking to helmut the production manager at Krieger, he suggested I use an obermeyer .308 match reamer considering the 175 grainy SMK rounds I would be using. After he confused me, I called a couple of reamer manufacturers at this point I was in reamer info overload. I was fortunate enough to get hold of Ted Brown and Art Luppino (cactus comet)that afternoon and they both straightened me out a little bit. So I decided to purchase an obermeyer .308 match reamer from pacific tool and guage for a couple of reason. One Krieger uses obermeyer reamers to short chamber there barrels two after talking to boots obermeyer he explained that he designed that reamer specifically for the M118 projectile which is a 173 grain pill and close to the 175 gr smk's. Dave at ptg tried talking me into a match .308 reamer. I would imagine because he had them on the shelf ready to go out the door as opposed to having to make thnx obermeyer. The two features that are different that stick out in my mind is the obermeyer has a 2 degree cone angle and a little longer throat for the longer round. Now back when inwas taking notes I could tell you what the cone angle was but I simply forgot. It's lime studying for a history test you learn it then dona brain dump. I never like wasting my time on middle eastern or European history when I could have been taking another math class but that's an entirely new chapter. Bottom line is I would like to think I Taylor my chambers to my round. Some way have a differing opinion and a great deal more knowledge about finish reamers than I do but they are not cutting my chambers or shooting my rifles. There are only a few finish reamers to choose from for this rifle. .308 .308 match obermeyer match. On a side note the obermeyer ptg made for menwas the first one he made in several years. I don't know if there are to many left out there but they can be had.

Gee wiz I wonder who chambered noexperts barrel? Ha ha . You better like it! That barrel was twisted on with love bruda.
 

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Most of the differences with the various chamber reamers are in 2 places:

1) The diameter of the chamber neck.
This is to make that section 'just right' for specific size of the loaded cartridge neck.
This requires making sure that the cases will work correctly in the chamber.

2) The length and diameter of the un-rifled section from the end of the neck to the beginning of the rifling.
This is tailored to suit the shape & dimensions of a particular bullet.

The benefit of doing this type of chamber selection is probably quite small - I guess maybe a max of 1/4 moa accuracy improvement.
Most shooters wouldn't be able to tell the difference, but for a serious competitor it could mean 1 more point or X, and be significant.
Having a special chamber usually requires more effort for reloading & buying components.

Jay Kosta
Endwell NY USA
 
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