Questions about military 300WM chambers.

Bob2650

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Minuteman
Questions about military 300 Winchester Magnum chambers. Both vintage and current.
Recently came across a box of 400 pieces of FC 08 300WM brass I bought at least 10 years ago. After sizing with a Redding body die it will not chamber. I will need to buy a Larry Willis collet die to use this brass in a normal 300WM chamber. Out of curiosity I measured a few pieces of this brass and found that they must have been fired in a very loose chamber with most of the body some 3.5 thousandths larger in diameter than the SAAMI max allowable spec. No indication of over-pressure was found with the primer pocket, base or belt diameter.
Just above the belt the brass measures 0.518". Allowing .001" for spring back the chamber probably is something like 0.519" in diameter. SAAMI spec for that spot is 0.5136" with a tolerance of +.002" and I found the same amount of oversize at .2" below the shoulder. The base of the shoulder was in spec.

So here are the questions:
Are overly sloppy chambers like that the field fix for over-pressure rounds in extreme heat? This ammo was loaded in 2008 so they were MK248 mod 0 with the 190 SMK .
I have never heard of such a reamer so I suspect someone was using a tool that could have been anything from a stick with sandpaper wrapped around it to a flex-hone from Brush Research. This explains the fire once and toss policy.

Bonus questions: If there was such a reamer, what is it's name or nomenclature and where can I get one?

Last questions: Do some of the MK13 rifles and other vintage rifles have such a loose chamber? Exactly which reamer was used to build the MK13? Perhaps the standard A191 is not the correct reamer for the MK13 after all? Is the military version of the reamer for the MK248 mod 1 also extra loose like this?

Probably too many questions,
thanks in advance,
Bob
 

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A191_diagram.jpg
 
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nn8734

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  • Feb 26, 2013
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    Questions about military 300 Winchester Magnum chambers. Both vintage and current.
    Recently came across a box of 400 pieces of FC 08 300WM brass I bought at least 10 years ago. After sizing with a Redding body die it will not chamber. I will need to buy a Larry Willis collet die to use this brass in a normal 300WM chamber. Out of curiosity I measured a few pieces of this brass and found that they must have been fired in a very loose chamber with most of the body some 3.5 thousandths larger in diameter than the SAAMI max allowable spec. No indication of over-pressure was found with the primer pocket, base or belt diameter.
    Just above the belt the brass measures 0.518". Allowing .001" for spring back the chamber probably is something like 0.519" in diameter. SAAMI spec for that spot is 0.5136" with a tolerance of +.002" and I found the same amount of oversize at .2" below the shoulder. The base of the shoulder was in spec.

    So here are the questions:
    Are overly sloppy chambers like that the field fix for over-pressure rounds in extreme heat? This ammo was loaded in 2008 so they were MK248 mod 0 with the 190 SMK .
    I have never heard of such a reamer so I suspect someone was using a tool that could have been anything from a stick with sandpaper wrapped around it to a flex-hone from Brush Research. This explains the fire once and toss policy.

    Bonus questions: If there was such a reamer, what is it's name or nomenclature and where can I get one?

    Last questions: Do some of the MK13 rifles and other vintage rifles have such a loose chamber? Exactly which reamer was used to build the MK13? Perhaps the standard A191 is not the correct reamer for the MK13 after all? Is the military version of the reamer for the MK248 mod 1 also extra loose like this?

    Probably too many questions,
    thanks in advance,
    Bob
    Contact LRI. Chad has a mk248 mod 1 reamer, which was used for all mod5 and mod 7s. The A191 was used in all previous iterations of the mk13 family.

    I can tell you that my mod 5 launched 220 smks at 2970 on average w/my mk248 mod1 clone load when i had it. Very accurate, no pressure signs.
     
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    Bob2650

    Boomer
    Minuteman
    I have that same print. All the information I have found is that this is the exact A191 chamber reamer that was used on the military sniper weapons. The Z dimension .5141 in the top right corner would have to be something like .519 in the chambers these 400 rounds were fired in.
    That is why I was hoping an armorer or gunsmith might throw some information on how those chambers got big enough to leave brass like that.
     
    Last edited:

    Bob2650

    Boomer
    Minuteman
    As a civilian there are plenty of larger cartridges and I do not feel the need to mess with my 300WM chambers. They burn barrels fast enough as it is. The energy level is close enough to the max my usual range allows for the steel targets. I would love to see the print for a mk248 mod 1 reamer. I believe they have longer throats, not wider than SAAMI in the body. Fat 300WM brass is nothing new and many people have bought once fired mil surplus brass with the issue. I think the oversize chambers are not rare on military sniper rifles.
     

    Bob2650

    Boomer
    Minuteman
    I have discovered that one of my old Hornady 300 Winchester Magnum full length sizing dies sizes close enough to all the way down to the belt that I can chamber that brass in all three of my 300WM rifles now.
    That die set was put away in storage while I searched for the "perfect" die set. I ended up with Redding as I shifted to searching for a "good enough" die set but the Redding does not size all the way down to the belt, close but not close enough.
    Problem solved, this batch of 400 pieces of the military version of Federal Gold Medal brass that is super consistent in weight and neck thickness has been saved from the scrap bucket without what looks like the incredibly tedious use of that collet die.

    I will be using this die for the first resizing only then switching back to the Redding as the old Hornady die works the necks hard enough I will want to anneal every time I use it. I put a 7mm button expander/decapper stem in the Hornady 300 WM sizer die to fix dents in the necks and will be using the usual mandrel to expand the necks.
    I will quit right there because this is a collectable rifle chamber thread and we have a different place for reloading/brass prep threads.:)