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Thread: Something strange, can anyone explain?

  1. #1
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    Question Something strange, can anyone explain?

    The other day I was out at the range working up loads for my 6 mm BR and had something strange happen. The rifle's got a 25 7/16" (don't ask it's a long story) PacNor "polygonal" barrel, on an old Ruger 77 action that's been a .22-250, a .243 twice, and now a 6 BR. I'd just fired some loads with 35.0 gr of AA 2520, CCI 450's, and 55 gr Nosler Ballistic Tips in Lapua cases with no problems, so cleaned the barrel and went to some loads with 8208 from 32.0 to 33.5 gr with the same components except for WSR primers and had no problems. I cleaned the barrel again then went to some loads with Benchmark and but otherwise the same as before. The first round with 32.0 gr of Benchmark was fine with normal bolt lift, and normal looking primer. The next two rounds gave very stiff bolt lift with slightly higher velocities than the first round, but again normal looking primers. Since this load is only a grain above what Hodgdon lists as a starting load and below what I'd tried with 58 gr Hornady V-Max's so I was a little baffled.

    Being baffled never stops me though so I tried the next charge increment, 32.5 gr and had stiff bolt lift with the first two rounds, with the third round giving normal bolt lift with an average MV of 3550 fps for the three rounds. Okay now that's strange to my feeble mind, but things got stranger. None of the primers looked hammered, and there was no cratering at all. WSR's tend to crater without too much persuasion in my experience anyway. I fired the 33.0 gr and 33.5 gr loads next with normal bolt lift on all six rounds fired, primers that looked about like those on the lighter charges, and only slight cratering on one primer with the 33.5 gr charge. Incremental velocity increases were fairly even from 32.0 to 33.5 gr.

    Temps were in the low 80's and the bench was in the shade under a roof.

    Does anyone have a reasonable explanation of why lower velocity/pressure loads in cases that had all been sized the same (FL sized in a Redding bushing die to give ~0.002" shoulder bump) and all chambered with no unusual effort produce stiff bolt lift while higher velocity/pressure loads don't? The necks were all turned and sized 0.263, and the chamber was cut for a no turn Lapua case, so tight necks weren't the problem either.

    I'm stumped.

  2. #2
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    Larry,

    Have any unexplained pressures on previous range visits with that Poly barrel...?

    I seen some strange things from a couple Polygonal barrels, .224 and a 6mm... Touchy / tweeky to load for..
    I perfer traditional rifled barrels myself.

    cale

  3. #3
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    case length??

  4. #4
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    This is interesting. A friend is seeing the same thing with a .20 Vartag Turbo and we haven't 'cured' it yet. The Lapua brass didn't fire form well on the first firing. The case necks were turned, but you couldn't slip a bulllet in a fired case neck. The brass has been re-worked now, but we haven't had a chance to shoot it again. We will also weigh instead of dump the charges. - nhk

  5. #5
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    There was another thread where someone reported something similar, namely, everything fine with a 6BR until Benckmark powder (published load) was tried.

    http://benchrest.com/showthread.php?...re-Need-Answer

    The responders went all over the map trying to diagnose what was wrong with Roland's rifle, just like they're doing here. My point was that since the only time a problem shows up is when you use a published Benchmark load, just don't do that.

    FWIW

  6. #6
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    Thanks for the replies gentlemen. I'll check the case length, but believe it's okay - seeing and believing are completely different things though. I've experienced this once before with a .223 Rem that showed pressure signs with moderate loads and nothing with some loads that were pretty hot except a little primer cratering.

    Strangely the hotter published loads with Benchmark were fine, although Hodgdon's max of 33.5 gr is listed as compressed in RP cases although it's nowhere near compressed in Lapua cases. I'm not seating as deeply as Hodgdon shows either.

    PacNor's "polygonal" barrels aren't truly polygonal more like normal rifling with a 5R profile if I understand things correctly and my eyes are working. I haven't had any problems since a bad chambering job on this barrel was cured by cutting off the old chamber and rechambering (different smith).

    I'll report back when I get the cases measured and another trip to the range made.

  7. #7
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    Quote Originally Posted by Larry Elliott View Post

    The necks were all turned and sized 0.263, and the chamber was cut for a no-turn Lapua case, so tight necks weren't the problem either.

    I'm stumped.
    This is not an answer to your pressure concern but why did you turn the necks to 263 for a Lapua no-turn chamber?? Isn't a Lapua no-turn for a 6BR about 272/273?

  8. #8
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    I've had this happen with a PPC. But, since it didn't group at the lower charge, I kinda dismissed the whole thing. Once I got up to where it shot OK, the bolt was fine. Maybe I shoulda given it more attention.....

  9. #9
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    Quote Originally Posted by JerrySharrett View Post
    This is not an answer to your pressure concern but why did you turn the necks to 263 for a Lapua no-turn chamber?? Isn't a Lapua no-turn for a 6BR about 272/273?
    The previous chamber needed a 263 sized neck, and it offended my delicate sensibilities to dump 300 Lapua cases with thinner necks. With my eyesight the way it is it doesn't make much if any difference in accuracy, just shorter case life likely.

  10. #10
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    OK- Some powder, bullet, case, and chamber/barrel setups will give these
    kind of problems due to airspace in the case with lighter loads.
    As soon as loads were heavier and airspace less or none, everything ok.
    Airspace with some combos can lead to small SEE incidents.
    Secondary Explosive Effects. Can also be major problem if loads
    to small with wrong powders. Some of these incidents have been seen
    on pressure traces, with a second pressure spike, but narrower than
    the main peak pressure curve at the beginning of ignition.


    In all of my big bore 12ga FH and my wildcat work I absolutely
    do not load with airspace, due to huge amounts of powder used.
    Might be a bomb if it happened in our big cases....ED
    Last edited by hubel458; 07-11-2011 at 01:15 AM.

  11. #11
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    Quote Originally Posted by Larry Elliott View Post
    The previous chamber needed a 263 sized neck, and it offended my delicate sensibilities to dump 300 Lapua cases with thinner necks. With my eyesight the way it is it doesn't make much if any difference in accuracy, just shorter case life likely.
    Thats about what the commercial maximum chamber/minimum cartridge gap is anyway.

    It will be interesting to see if better-than-commercial (Lapua) brass lasts longer with this much movement. Sizing just the part of the neck with a bushing die will still probably help accuracy..

  12. #12
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    Jerry, if I can get half inch groups from a live varmint rifle with the way my eyes are I'm happier than a fox in a chicken coop. I've tried just sizing part of a neck for accuracy, but found that the necks crack right where the little shoulder from the sizing on the neck is. I've got to go measure the length of those cases.

  13. #13
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    I checked the case lengths on all 50 fired cases in the box and they were almost all just slightly over 1.560", so they're now all trimmed. That makes me feel a little better, but even after FL sizing they were only maybe 0.005" long which isn't enough to cause a problem. All the "problem" rounds chambered easily too.

  14. #14
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    Larry,

    This perplexed me for some time because this was a fairly normal occurance with V133 in a PPC. What a lot of people will find is when they got above 29.0 grns or so the bolt lift would be tighter and then around 29.5-6, the bolt lift would go back to normal. How this was explained to me, was there are two things that can occur that causes this. The first one is mentioned above in hubel458 post. The second seems to be the most common and occurs with cases with less body taper.

    There is a pressure range where the case will smack against the boltface harder because the pressure is just not quite there to slam the brass case against the chamber wall enough to grip it (so to speak) as it does with a slightly higher pressure. I don't know if I explained that well enough but it is fairly repeatable. It is really noticeable is polished chambers that have less wall grip during firing.

    Hovis

  15. #15
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    Check for a "carbon ring" buildup in the forward neck area. You can find it easily by buffing a loaded bullet with your favorite "neck wiper", chamber the round and look for burnishing on the bullet that doesn't resemble land marks.

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