7mm STW - Stuck cases, at wits end

Discussion in 'Gunsmithing' started by One_Duck_Limit, Jul 24, 2011.

  1. One_Duck_Limit

    One_Duck_Limit Well-Known Member

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    I've got a 7mm STW, actually it's my buddies, that the cases stick after being fired. The bolt lift is fine, not stiff at all, with the bolt completly up we have to tap the case out with a cleaning rod. Last Saturday at the range we tested 7 different loads, 180 Berger VLD .010 off the lands, Federal 215M primer, 73 - 76 grains Retumbo in .5 grain increments. Everything was shot over a chronograph to verify velocities. At times the stuck case seems completely random. The load with 74 grains stuck, the load with 76 grains did not. After the case is tapped out of the chamber you can see an very obvious mark in front of the belt where the case rubbed against the chamber. We have checked and double checked the headspace, replaced the bolt handle to ensure primary extraction and replaced the extractor.

    Any ideas what is wrong with rifle? Could the chamber be rough or out of round?
     
  2. Camshaft

    Camshaft Well-Known Member

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    The chamber needs to be polished. It happens. Unless you have done that already I would give it a try.
     

  3. IdahoCTD

    IdahoCTD Well-Known Member

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    If it's a factory chamber the back of the chamber can be oversized also. For that matter even a custom chamber could be oversized if the back of the reamer wasn't straight when it was pushed in the barrel. What do the cases measure in front of the belt? Are there ring marks in the cases?
     
  4. LewisH

    LewisH Well-Known Member

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    The chamber is screwed up. Don't fool around with it, just send the rifle back to whoever 'smithed it, or to the manufacturer if it's factory original.
     
  5. One_Duck_Limit

    One_Duck_Limit Well-Known Member

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    Right in front of the belt it measures .510. There is a ring around the case where it appears to get stuck and it measures .515 at that ring.
     
  6. One_Duck_Limit

    One_Duck_Limit Well-Known Member

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    Here is a picture of two fired cases. You can see where the case appears to get stuck in the chamber about .050 in front of the belt.


    [​IMG]
     
  7. IdahoCTD

    IdahoCTD Well-Known Member

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    Sounds like a oversized chamber. When a friend and I tought ourselves to chamber rifles we used the tailstock to push the reamers in and had several oversized chambers, one was a STW just like yours and the brass came out right at .515-.516. I had the same problem with sticking brass. We later correct the tailstock but also went to a floating reamer holder. Is it a factory chamber?
     
  8. One_Duck_Limit

    One_Duck_Limit Well-Known Member

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    No, this is not a factory chamber or a factory barrel. What are the options to fix it? Set the barrel back and re-chamber? How much setback would you recommend?
     
  9. IdahoCTD

    IdahoCTD Well-Known Member

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    The amount of set back would depend on how the threads were cut to a degree and how big the back of the chamber actually is. If there is a relief cut for the threads I would set it back at least the length of the threads. If there isn't a relief cut and you or a gunsmith feel confident you can get the cutter back in the threads then you can possibly go shorter. If it was me I'd cut the threads off and recut the threads to make sure it cleaned the chamber. I'd get a 7mm range rod to dial the barrel back in off the bore.
     
  10. One_Duck_Limit

    One_Duck_Limit Well-Known Member

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    Thanks for the advise. Is there a way to make a mold of the chamber to compare against SAMMI specs?
     
  11. IdahoCTD

    IdahoCTD Well-Known Member

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    Brownells sells the stuff to do chamber castings. It's best to remove the barrel and then you plug the bore to do it.

    I haven't measured my reamer but if I remember right it's suppose to be .512 right in front of the belt. So yours is probably at least .516 and I wouldn't be surprised if it was .518 or more. The amount of set back will change with the size of the chamber. I'm pretty sure a STW/8mm Mag is .512 at the base and .495 at the shoulder. So the more it is oversized the more you'll have to set it back to clean the chamber since the case has very little taper per inch. If the reamer goes in true with the bore it will be less than .001 over the reamer size.
     
  12. One_Duck_Limit

    One_Duck_Limit Well-Known Member

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    This may be a dumb question, but why would an oversized chamber cause the case to stick?
     
  13. IdahoCTD

    IdahoCTD Well-Known Member

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    I'm not 100% sure why it does it with a oversized chamber. I think it's because the brass extends beyond it's normal range too many times and becomes less elastic (work hardens). So upon firing it wont spring back off of the chamber walls. I would assume new cases are less likely to do it?
     
  14. One_Duck_Limit

    One_Duck_Limit Well-Known Member

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    Yes, new cases are less likely to stick, 1 out of 20 maybe. Even then the bolt would extract with a bit of effort. Fireformed cases are 1 in 3 and they have to be tapped out with a cleaning rod.