Big magnums developing pressure signs as barrel warms?

Discussion in 'Reloading' started by Marine sniper, Oct 2, 2011.

  1. Marine sniper

    Marine sniper Well-Known Member

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    I have an issue with my 30-378 that I am having a hard time getting a handle on. The rifle is a custom long range rig

    Hall G
    34 in Pac-nor 3/3 1.25in bull barrel
    .333 neck.

    I am shooting 240 SMK at around 3050 fps when everything goes right...but I get weird pressure signs out of nowhere. I have had this happen with powder charges from 103-108 (H50BMG) I went out today and shot 13 rounds and everyone (except one) was faster then the last one. I shot a 5 shot group, let it sit 5 minutes, then a 3 shot group, let it sit 10 minutes and finally another 5 shot group. The velocities ranged from 3038-3106. Anything above 3070 or so and the bolt gets a little sticky and the ejector leaves a nice shiny reminder the pressure is too high. The ES seems very high considering each piece of brass is fully prepped, annealed, necks cleaned, trimmed, etc. The same procedure in my other rifles will yield single digit ES's.

    I have fought this issue every time I have taken this rifle to the range. I have reduced the load from 108 down to 103 and beleive it or not it does not seem to make much of a difference. Temps when shooting have been from 70's to high 80's, again this did not seam to matter either. This time to the range I weighed each powder charge on two different scales (103 grs.), measured every neck clearance (I run .002-.0025) and every seating depth (+ or -) .004 total variance. Nothing obvious other than as the rifle got warm the velocities went up until things got unhappy.

    I called Hodgens and asked them if they have had a bad batch of H50BMG and they say no.

    I remember when I was in the Marine Corps our first generation 50 cal's would do this. The bolt would get sticky as they warmed up, but this was with a pretty crappy rifle and military ammo, none the less it did happen.

    My next step is to try some Retumbo or at least another powder.

    Thanks,
    John
     
  2. Marine sniper

    Marine sniper Well-Known Member

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    Here is a pic of todays 100 yard target. The top row is my short barrel .308 pack rifle with 200 NAB's. Too slow of powder (RE17) but good accuracy potential for a 20 in barrel less than 7 lbs rifle.

    Not too make excuses for the fliers; but I had a group of guys with their M-14's, etc shooting rapid fire right next to me. It is hard for me to shoot tight groups when I have BANG, BANG,BANG right next to me. I am sure the 30-378 is a 1/4 MOA rifle when I can do my job. It should really shine at long range when I can get this pressure thing worked out.
     

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  3. J E Custom

    J E Custom Well-Known Member

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    Not sure , but it sounds like it is getting more and more fouled.

    Clean it to bare metal and then shoot it one time and clean. repeat the process until you have shot
    6 or 7 times and cleaned each. do this through your chronograph and see if it makes a difference.

    Also when you close the bolt on a round shoot it as soon as you can so it doesn't have time to heat up the round. The big 30 will heat up the chamber after only 2 shots.

    Just something to try.

    J E CUSTOM
     
  4. royinidaho

    royinidaho Writers Guild

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    What J E said?
     
  5. Marine sniper

    Marine sniper Well-Known Member

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    I will give it a shot. Thanks.
     
  6. Trickymissfit

    Trickymissfit Well-Known Member

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    those big mag cases don't like to be down loaded much, and really like to be loaded close to max. I'd try 111.0 grains of AA8700. Those cases like real dense loads
    gary
     
  7. Marine sniper

    Marine sniper Well-Known Member

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    Gary (from yellow bullet?)

    AA8700 is faster than H50BMG. The only slower powders Hodgens lists is US869 and VV 20N29. I am at approx 90-95% density now. I "think" JE might be on to something, the barrel has 85 rounds through it and cleans very easy, but his idea sounds plausible, particularly when I review my date. I will give it as shot and see what happens
     
  8. Trickymissfit

    Trickymissfit Well-Known Member

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    Many years back I knew a guy that downloaded a .378 to reduce the recoil. Didn't kill him, but did put him out of business for awhile. Interestingly the the rifle wasn't destroyed, but the barrel and his collar bone was.

    Your load is still within the window that the manual says is safe, but my thoughts evolved around the thought that each and every barrel seems to have it's own quirks. Have never used or even been around the H50BG stuff to be honest with you, and everybody I ever knew that did that case used H870 and AA8700. Knowing who you are, and if memory serves me right, it seems that you are shooting without the freebore in the throat. I suppose it's possible that the throat is really foulded with burn powder. So pressure spikes will be a virtual square (in OTW very quickly increase). I've never noticed pressure spikes with fouled barrels, but just accuracey problems. Still I don't shoot that case (I doubt my body could stand much of it), and get all the pain I can stand out that .450 Marlin. I turned 66 the otherday, and just don't heal up as fast as when I was your age!

    gary
     
  9. Marine sniper

    Marine sniper Well-Known Member

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    Gary,

    This rifle kicks bout like a 308. I have never put it on a scale, but it is over 20 pounds for sure. You are correct, no freebore. Bullets are kissing the lands. I may try backing them off a little.

    Thanks,
    John
     
  10. Trickymissfit

    Trickymissfit Well-Known Member

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    good luck, but be carefull! Any idea whatkind of pressure you should be seeing before things went wrong? The 111 grains of AA8700 is about 59K psi
    gary
     
  11. Marine sniper

    Marine sniper Well-Known Member

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    No, it would only be a guess.

    I leave Saturday for an elk and deer hunt in Idaho, so no more time for testing. The 30-378 is staying home this year.
     
  12. Trickymissfit

    Trickymissfit Well-Known Member

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    is that the rifle you shot the grizzley with last year? I'll probably head south for a deer hunt next month, and hope to be back in time to do some serious pheasant hunting.

    gary
     
  13. Marine sniper

    Marine sniper Well-Known Member

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    I did not shot a bear last year. I have never hunted with the .30-378, but should next year.

    John
     
  14. Trickymissfit

    Trickymissfit Well-Known Member

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    I'm refering to the one in the picture. Thought you got it a year ago. Remember seeing it on Y.B.
    gary