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Thread: 300 win mag issues

  1. #1
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    300 win mag issues

    I recently put a new barrel on my 300 win mag and took it to the range. While there I fired 8 shots and on the 8th shot I had full case head separation. I took it home assuming my load was bad or something. Once home I checked the headspace again with forster and ptg gauges and went out and got a box of factory ammo to try that, well today I took it back out and it made it 4 shots with perfect brass and no pressure signs and the 5th one split. Chamber is clean and polished the bore is smooth and pretty. Anyone have any ideas why it would do this? I have 2 others I put together the same way and had no issues.


    Edit: I also tried changing the bolt face and resetting the headspace but... same story

  2. #2
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    Are these reloads? If so you have 2 headspaces to worry about with a 300. The first and most obvious is the belt, which is the true headspace for the cartridge. The second is the shoulder just like in most other cartridges. I have run into this myself. I have a Winchester Model 70 that has an extra long space to the shoulder. Setting the resizing die as directed led to early case failure. Get a bump gauge for the 300 and start checking fired brass against your reloaded brass. You will likely be able to solve your issue with an adjustment to your resizing die. Set the resizing die to just bump the shoulder back 0.001-0.003.

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    Thank you for the reply however I tried that this afternoon with a piece of brass that survived the first firing I bumped it back .002 and fired it. I result was .003 worth of growth and a large split above the belt. I’m really stumped I don’t see how it can be the headspace since I used the shoulder this firing. But it has to be headspace right?

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    What tools are you using to check bump/shoulder headspace? If the separations are not being caused by headspace then you must have some of the worst brass ever made. One other thing to check, and this is simple. Take a piece of brass, unloaded, and put a piece of scotch tape or any tape that is about 0.003 thick and place it over the base of the case. Trim it. Now put it in the chamber and see if the bolt will close. Don't force the bolt to close as you can crush or distort the tape. This is easier to feel if the ejector has been removed as the ejector is going to push on the tape the whole time. If the bolt closes normally then the true headspace is not correct and you will need to reset it or check it again with a go gage.

  5. #5
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    Where did you get your brass?
    One Cannot Be PC And Be Intellectually Honest!

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    I used a RCBS precision mic for 300 win mag to check the bump and I will try the tape. However I have reset the headspace twice now with two different sets of gages. Is the tape going to measure something different? I have never done that before and I’m interested in the approach

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    Quote Originally Posted by Robinhood View Post
    Where did you get your brass?
    the brass I have been using for this rifle is all virgin Norma brass from midway.

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    This die has helped be with case head seperation on my 7mm rem mag. Works with all belted magnums.

    http://www.larrywillis.com/
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    If the barrel/chamber is properly positioned then there will be no more than 0.003" between the case head and bolt face, this is your primary headspace with a belted magnum. Placing the tape on the case base will allow the case to act as a no go gauge and if your bolt closes then the primary headspace was set incorrectly. This is assuming that the Norma cases are made properly. Do you still have some of the virgin Norma brass? If so using your RCBS tool measure the shoulder on it and tell me what difference you observe. I have both Winchester and Norma virgin brass on my loading bench. The Norma brass is 0.004-0.005 shorter than the Winchester brass. The Winchester brass chambers in a new 111 without any issue. The Norma is going to have to grow a lot to catch up. When I was having this issue like you, I was advised to lubricate the new cases before firing to reduce the case stretch just above the head. I have done so since then and have never had any more issues with new brass. I use a very light but uniform application of imperial die wax on the case body. None on the shoulder or neck. The lube supposedly lets the brass stretch uniformly across the length of the case body. I don't know if it does or not, but it did help. One other thing you might try if you have virgin brass left over (works with fired cases also). Straighten a paper clip and then bend over the last 1/8-1/4 of an inch of wire so that the wire is now L shaped. Reach into the case with the bent section and move the tip up and down where the cases have been separating. You should be able to feel the weak spot with the paper clip if it has already formed.

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    Here is a link to the thread I started on another forum about these issues. Read it paying particular attention to the replies from CatShooter. At first you may think we had different problems, but read on.

    http://forum.accurateshooter.com/thr...space.3860274/

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    Well I checked the headspace with the tape method and got the same results my Forster nogo gage gave me. However the link you posted has some good information and I can’t help but think lubing the brass as it states may help. Because both sets of headspace gages say it’s good so I’m left to think the case expanding issue is the ticket. I’ll try ASAP and report back

  12. #12
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    Since the tape gave the same result as your Forster headspace gauge then the belt on the Norma brass is not the reason for the separation. It most likely is a long stretch for the brass that is causing the issue. If not the stretch then the only thing besides bad brass left is a raised burr in your chamber which you have checked. Good luck.

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