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Thread: Wanted: 300 WM Chamber Dimensions

  1. #1

    Default Wanted: 300 WM Chamber Dimensions

    Hi there;
    I tried surfing for the Chamber Dimensions and tolerances.
    Lots of cartridge dimensions...everywhere. Those don't tell me what I am trying to determine, looking for the machine drawings and specs.
    I will be making a chamber cast, but I don't know what clearances are normal, good or bad. Anybody have this stuff laying around, or a link? In my world .002" is enough to throw a cat through, but I am sure there's more slop in even a good chamber.
    Additionally wondering how straight a barrel should be. This also comes in degrees....

  2. #2
    Join Date
    Jan 2005


    I can give you maximum case dimensions for the 300 Win Mag.

    These are not the same as chamber dimensions but from these you can determine chamber dims from these or very closely. Obviously any 300 Win mag chamber should accept any 300 Win mag brass, so the shamber must be slightly larger. What is slightly? About .0010"-.0020". (one to two thousandsths of an inch)

    Your camber cast will tell you all you need to know. Cerrosafe is a very low temp melting point and very low shrinkage when cooled. The dial caliper will give rough quick dims you'll ned a mike to get exact numbers.

    Push a jag with a patch on it from the front and stop just in front of the rifling beginning, you want to see the rifling on the cast. This will also give you the throat dims. Pour the melt until it fills the belt recess of the chamber. Wait about 3 minutes then punch it out.

    Rim diameter-------------.5320"
    Belt diameter-------------.5320"
    Base of body diameter----.5130"
    Shoulder diameter--------.4891"
    Neck diameter------------.3397"
    Rim thickness-------------.0050"
    Length to belt ------------.2200"
    Length to shoulder--------2.1959"
    Length to neck------------2.3560"
    Length to mouth-----------2.6200"
    (max case length)
    Shoulder angle-------------25 degrees

    These are from Ken Howell's excellent book;
    "Designing and Forming Custom Cartridges"
    This book is a work of art and would be a great addition to any gun enthusuast library, but a necessity for anyone who harbors ideas of designing his own wildcat or just modifying a current caliber to boost its performance. Ok, Ken, how's that?
    Is there nothing so sacred on this earth that you aren't willing to kill or die for?

  3. #3

    Default MAximum Case Dimensions

    Hi Murphy and thanks yet again!
    Yeah, case dimensions I have but the clearances I did not. I know well .002" is plenty to get that cartridge in and out of the chamber, and at a guess probably what I would allow, possibly less because these are the maximum.
    My fired brass where the heads did not separate at the belt were expanding .0095" just above the belt which seemed completely out of line to me, but I just don't know. I recall reading a specification that I was appalled at... +- .007" on some chamber... think it was a SAMMI Spec. Yikes!
    Anyhow, thanks again, and any thoughts of what a straight barrel should look like? There is straight, and straight...after stress relief, then further machining, rifling, welding of hardware, stressed are re-introduced. TIR .030" over the length of a 26" that straight? And should I go ahead and straighten it or leave it? Consistency being the greater part of accuracy...
    I dunno...
    Thanks again,

  4. #4
    Join Date
    Feb 2008


    OK, this sounds like a headspace issue. To try to measure headspace with a depth mic, or other meathod would be difficult and likely lead to poor accuracy. If someone were to tell me a chamber was to deep based on such methods I would be concerned as to there tools and ability. The accepted way of measuring headspace is with go/nogo-gage's. Go/nogo-gage's are cheap and if you can gently close the bolt on a nogo-gage then there is no room for your rifle builder to argue, the chamber is to long.

    John Hinnant, Recommends that headspace to set to 0.001" longer then a go-gage. I am sure other gunsmiths have there own ideas. Most go and no go gages only differ by 0.004 to 0.006", SAAMI spec. generally 0.007".

    If you really don't want to get a set of gages then add shims to the back of an unfired case till the bolt will not close gently on the case. Do not slam the bolt down it will compress the case or gage and close when it should not. Measure from the belt to the head of the case including the shims. This should not be more then .220+.007=.227".

  5. #5

    Default Headspace

    Hi Bandhmo;
    Yeah, for sure lots of headspace. Pretty danged sure the chamber reamer got buried in the barrel. pretty difficult to get good solid measurements from a fired casing, but I would venture .040" of stretch comparing an unfired factory round and the fired case. Enough to cause complete or partial case separation of every handloaded cartridge and severely stretch the new factory brass in the same exact area immediately forward of the belt.
    The other Headspace....004" between barrel and action, casings sized to protrude .003" leaving .001 for clearance.
    Chamber throat also bottomless, bullet seated to approximately ( Tryin' for) .001" short of imprinting rifling through prussian blue.
    So, yeah, I don't believe I left much to chance. Piece is back at factory, I was a bit cross with them. I will know a bit more when I cast the chamber and I think Murphy is probably correct in that .002" Clearance all around in the cartridge dimension being about right. I am a different breed of machinist j just don't know the mechanics and fits in firearms. Other than that I can run with the best, or at least I knew enough to insult the manufacturer with four place measurements. Least I could do on good ground.
    Anyhow, thanks for the input! I'm pretty new here and it's a great bunch!

  6. #6
    Join Date
    Jan 2007
    Southwest Alaska


    Call Pacific Tool & Gauge and get a copy of "The Gunsmiths Book Of Chamber Prints". Cost you $80.00.
    Great spirits have always found violent opposition from mediocre minds. The latter cannot understand it when a man does not thoughtlessly submit to hereditary prejudices but honestly and courageously uses his intelligence. Albert Einstein

    Better living through chemistry (I'm a chemist)

    You can piddle with the puppies, or run with the wolves...

  7. #7
    Join Date
    Feb 2008


    All of the things you mention sound like excess head space but many of the numbers you state I do not understand. I understand that you are a machinist and are used to working to close tolerances. However, I do not know what you do or do not know about Gunsmithing so please don't feel I am talking down to you. I myself have some machining experience and am trying to learn more about gunsmithing as a hobby. Perhaps the confusion comes from a different meaning of headspace.

    On a belted round like the 300WM headspace is set by the front edge of the belt. The distance to the shoulder or datum diameter is a specification but is not headspace, as it is in a rimless/beltless round. The headspace of your rifle is the distance from the bolt face to the ridge that the belt of the cartridge rests against. So I am not sure what you meant by other head space, though I would guess you are talking about bolt to barrel clearance which is a factor in headspace but not the same.

    If the belt headspace is correct then you should not be getting separations. I am in no way recommending this but, if you were to fire a 300 WM round in a longer chamber rifle like the 300 weatherby, you would not get case separations. The shoulder would blow forward to meet the chamber, but the case would not separate as the belt would hold it firmly against the bolt (you might get splits in the neck but that is another issue).

    I my dimensions for the 300WM Chamber came from the Clymer Reamer Web page that I typed into a speadsheet. Clymer no longer has them up so I can't go back and double check. All of the dimension I have are about 0.001" bigger in diameter then Murphy's. This is not a surprise as the tightness of a chamber is dependent on intended use. A benchrest rifle will have a very tight chamber, while a dangerous game stopping rifle with have a relatively loose chamber to guaranty reliable feeding, even when hot and dirty. I am not sure of the SAAMI spec. but 0.002" cartridge to chamber clearance does not sound excessive in anyway.

    You are certainly correct to be mad at the builder of you rifle. This kind of thing is inexcusable. Especially Since Belted Mags are relatively easy to headspace, and all except the 450 Marlin use a 0.220" headspace. I am not sure what is happening with the throat being long as I seem to remember the 300WM not having free bore like the Weatherby line.

    As for doing A chamber cast I would recommend a product called Cerrosafe. I have not used it myself but it has an excellent reputation for easy use, low melting temperature, and dimensional stability/predictability. It is available though Brownells among others.

    Good luck and I hope it all gets worked out.

  8. #8

    Default Bandhmo

    Hi there and thanks again!
    Yeah machinist in everything but cars and guns.
    Cerrosafe is on the way. Shoulder of fired cartridge is pushing out about .040 compared to unfired factory. Firearm returned with comments.
    When it returns I will go over it with extreme scrutiny. Chamber cast, crown inspection, check barrel for straight and runout of bore to barrel and bore to chamber.
    I would hate to have me for a customer...
    Thanks again for the comments and no worries.
    This is war........


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