Excessive Throat Growth Question

Discussion in 'Rifles, Bullets, Barrels & Ballistics' started by royinidaho, Feb 27, 2012.

  1. royinidaho

    royinidaho Writers Guild

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    Not sure how to proper ask but, here goes.

    My favorite rig has dropped off in accuracy. I've used the same COAL (Cartridge Base to Ogive) since the barrel was new. Have 900 hot rounds down the old girl.

    Guilt edge accuracy has gone.

    How much increase in length of throat, from original, is a decent give up and redo point? Or is there something that can be done to squeeze a few more rounds out of her?

    As close as I can determine throat has increased 0.047".
     
  2. load

    load Well-Known Member

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    i'm having one of my rifles done right now. i'm cutting 1" off minimum is about 1/2 but i'm goin on the safe side.gun)
     

  3. Bart B

    Bart B Well-Known Member

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    With about .050" of throat wear at the bullet's ogive contact from where it started when new, that's about normal when accuracy drops off. But the worst part of this is the throat's eroded more than that further down the bore. That's the part that tends to gouge bullets and upset their perfect form so accuracy degrades.

    Usually, if one seats bullets .010" longer than throat contact in necks that aren't too tight so the bullet seats back a bit when the bolt's closed, accuracy will still continue to be great. Sometimes such that bullets need be seated .075" further out than when the barrel's new.

    What cartridge were you shooting? For some, 900 rounds to loss of gilt-edge accuracy is normal.

    And what do you mean by "hot" loads?
     
  4. royinidaho

    royinidaho Writers Guild

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    Thanks for the informative response.

    This rig has been ridden hard but not mistreated.

    270 Allen Magnum.

    Hot = 5 shots before having to retire cases due to no longer holding primer. No longer holding means when the primer falls out after hand seating the case goes in the bucket.

    170 bullet at 3400
    195 @ 3100
    150 @ 3650

    The cartridge is designed for a lifetime of taking game. I seemed to have used it for a practice gun with most shots at paper and rocks out to 1400+ yrds.

    I'll have much more respect for the next barrel.

    I can feel the rough bore ahead of the throat. Reduced the roughness considerably by thorough cleaning. Will give it good dose of Bore Tech Eliminator, seat things a little longer and see if I can milk a little more accuracy.

    The shame is is that she held a solid 1/2 MOA for well over 800 rounds. Count how many woofs cudda been done in instead of rocks...:rolleyes::rolleyes:
     
  5. GNERGY

    GNERGY Guest

    I was reading on Accurate shooter that sometimes you also get a buildup of carbon fouling in the throat area that copper cleaners don't really remove. I also swear by Bore Tech eliminator. They recommended using something more abrasive like JB Brobst to scrub the throat and the first 10 inches of the barrel. Most of the 600yrd to 1000 yrd benchrest shooters do that about every 150 to 200 rounds or when accuracy starts to drop off. Couldn't hurt to give it a try.
    What ever happened on your elk hunt with the pic of the 2 wolfs?
    Tarey
     
  6. grit

    grit Well-Known Member

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    Depends on the condition of the bore ahead of the throat. Sometimes a turn (.060) is plenty. In your case, I bet it's cracked and hardened for at least an inch, if not several. Gotta clean 'er up good, take it off, and have a look.

    You can chase the lands a little with your bullet seating depth. Might get you a few more rounds.
     
  7. Gene

    Gene Well-Known Member

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    Bunch of good tips here. My gunsmith will not re-chamber a barrel with a firecracked bore, says it is a waste of money. Even if the bore is reasonably clean, all you can expect (with a 6ppc) is maybe another 600 rounds before it is toast.
     
  8. Bart B

    Bart B Well-Known Member

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    If cases last only 5 shots then primers start falling out, consider what's happening. Peak pressure's close to 65,000 CUP (81,000 PSI). Cartridge brass starts extruding between 60,000 and 65,000 CUP depending on its metal composition. Your cases are extruding 'cause they don't spring back to where they started in the primer pocket area. So, you have mistreated the barrel by firing ammo in it that's got too much peak pressure that not only shortened the normal barrel life for that cartridge. And you've also mistreated the cases. For a comparison, 7mm Rem. Mag. barrels that shot that accurate back when they were popular in prone long range matches lasted for that many rounds before opening up a noticable amount. And the Allen version burns a lot more powder. Allen's web site states this about their 7 Mag.: "Limited barrel life, cases somewhat involved to form, must use ultra slow burning ball powders." I'm surprised you got barrel life that long.

    Such is life when folks are only happy operating their rifles with peak pressures in the small margin of safety range where cases and barrels don't last very long.
     
  9. bigngreen

    bigngreen Well-Known Member

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    Roy, did you catch a couple weeks ago one of the Ausies mentioning throat maintenance with KG 2. I Gave it a try on one of my WSM's this weekend and the results were good, you can feel a difference in the throat area. I've also heard that David Tubbs recommends running one throat conditioner bullet after a string, I think it's basically doing the same thing.
     
  10. royinidaho

    royinidaho Writers Guild

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    The down under guy's response was to a question I asked.

    I'll have to learn what K2 is???

    All I want is to get through this bullet testing phase. I learned something from your testing. Seeing as how NABs were sub 1/2 MOA I'll get the bore shooting them as best I can and compare the pointed bullets to them. At this point I think I have a concentricity problem. When things that that small its tough to get a consistent setup for measurement.

    I have a 'new' 270 AM barrel w/only 60 rounds through it. Hoever, its 28" and a much smaller contour.

    Bart B,

    I've known what I was doing from the beginning.:rolleyes: I just didn't expect the barrel end-of-life remorse to be so great. I really love to shot that thing. Pretty much addictive.:)
     
  11. bigngreen

    bigngreen Well-Known Member

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    Roy, I know I have a WSM barrel that the throat is visibly longer in and the throat is rough, it has close to 2000 rounds down it and I'm going to give that a try with the KG2 to see if I can restore some accuracy with the Bergers, the Accubonds and BT's shoot well in it still but the thin jacket bullets going fast started spraying all over the place at a little over a 1000 rounds. I'm starting to wonder if it's how smooth the throat is vs how long it is. I'm going to keep up with it in the current barrel which only had a few hundred down it to see if I maintain the accuracy longer.
     
  12. Bart B

    Bart B Well-Known Member

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    OK. Next time use this info and formula to calculate barrel life for a 1/2 MOA rifle from the get-go so you'll know what to expect:

    Barrel life's about 3000 rounds when there's one grain of powder for each square millimeter of the bore's cross sectional area. For a 7mm, it's bore area's 38.5 square millimeters; the 7mm-08 gets about that much barrel life. Double the powder charge with a larger case capacity and barrel life's cut by 75%; 750 rounds is all one gets at 1/2 MOA accuracy for rifle cartridges working at normal max loads in the 55,000 CUP range. With hotter loads, barrel life will be even shorter. It's an inverse square law that seems to work pretty well.

    How many grains of powder have you used with your 7mm AM's loads?

    PS: If the barrel's best accuracy is 3/4 to 1 MOA at the beginning, one will get 3 to 4 times the barrel life the above formula produces.
     
  13. load

    load Well-Known Member

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    id have to dissagree with him hes probably trying to sell a new barrel. the fire cracking on a barrel is usually with in the first 1/2" if you set the barrel back a an inch you have a passed the part of the barrel were the pressures and heat are the highest. from there the bore is usually just fine and will last another 1000 or more rounds. how ever not all barrel contours can be cut back 1"
     
  14. Boss Hoss

    Boss Hoss Well-Known Member

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    A Bore Scope removes all doubt! One look is worth a thousand "expert opinions"!