bullet holes in target. (not perfectly round)

Discussion in 'Rifles, Bullets, Barrels & Ballistics' started by 7man, Nov 11, 2009.

  1. 7man

    7man Active Member

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    Question for you guys. Today I was seeing how my 7 rum would like some 175 gr sierra gamekings with 88 and 89 grains of retumbo. It seemed to like the 88 grains with a 1.5 inch group (best I've achieved so far with the gun).
    Now with the 89 grains. I had a string of shots about 6 inches across the target and the bullet holes weren't perfectly round. All the holes with this load had a small rip outside the perfect round hole. The holes looked like the cut an out of tune bow makes shooting through paper. Is it possible my bullets are tumbling and causing this?
     
  2. Jim Oliver

    Jim Oliver Well-Known Member

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    Not tumbling (not yet, anyway), but wobbling a bit. Sound like the twist rate of the barrel is too slow to stabilize the bullets, at the present velocity.

    Jim
     

  3. Buffalobob

    Buffalobob Writers Guild

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    If the distance was 100 yards then try the 88 grain load at 300 yards.

    A 7 RUM should have a 1-9 twist barrel which should be fine to stabilize the 175 grain bullet. Most likely cause is that you have some initial yaw which will go away at about 200 yards and by 300 yards be settled down

    You might check the crown and see if it is dinged.

    I would suggest you try the 175 SMK, the Berger 180 and the Accubond 160. You may find that the rifle actually prefers a bullet that you don't like.
     
  4. Jim Oliver

    Jim Oliver Well-Known Member

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    "A 7 RUM should have a 1-9 twist barrel which should be fine to stabilize the 175 grain bullet. Most likely cause is that you have some initial yaw which will go away at about 200 yards and by 300 yards be settled down"

    Certainly worth trying.........I should have mentioned it.

    Jim
     
  5. yobuck

    yobuck Well-Known Member

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    buffalobobs suggestion is good advise. most senderos shoot better than that from what ive seen. several guys here have them and use 162 amaxes. try a different powder also, like 7828, or h1000. i use the 162 bullet and 7828 in a 7x300 and im happy with it. i have another that dosent like 7828 at all.
     
  6. BryanLitz

    BryanLitz <b>Official LRH Sponsor</b>

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    With all due respect, I have to disagree with Bob about these bullets settling down at longer range. It's not just imperfect bullet holes, they're shooting "about 6 inches across the target" also. That will not improve to acceptable groups at 200/300 yards.

    My guess is that the higher charges are damaging the bullets structurally which is causing the large groups and weird holes. This damage may be partially due to a rough factory barrel. Of course it's also partially due to the huge amount of powder and high velocities reached by the RUM case.

    I do agree that a 1:9" twist is plenty of spin for these bullets, so the problem is not lack of stability. My hunch is that your rifle is just too hard on the bullets at very high speed.

    There are two areas of bullet failure that might be possible (if it is a bullet failure problem). Either the exposed lead of the GameKing bullets is becoming deformed on firing. I've measured inconsistent BC's for lead tipped bullets as charge weight increases. I believe this is due to the lead tip deforming somewhat at greater accelerations. If this is the problem, you can solve it by going to any other non-lead tip hunting bullet.

    The other failure that's possible is at the jacket. This would be due to a long bearing surface scraping down a rough factory bore at high velocity causing rips/splits in the jacket. If this is the problem, you can solve the problem by sticking to the heavier jacket options like the Nosler Accubond. If this is a 'rough barrel' issue, I advise against the Bergers because their even thinner skinned than the Sierra GameKings.

    Best of luck. Please let us know if you figure out what the problem is/was.

    -Bryan
     
  7. elkaholic

    elkaholic Well-Known Member

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    I agree with Bryan, especially on the jacket issue! Thin jackets, high velocity, tight twist, rough bore, etc. are all recipes for bullet failure. I have seen it with my 30" 300 RUM with 180's. My guess is something like the Accubond will solve your problem. Good luck....Rich
    p.s. another possibility is carbon buildup in the throat. This too can tear up a bullet. If the accubonds don't work, try soaking the throat area with some Kroil for a couple of days and then clean and see what comes out?
     
  8. royinidaho

    royinidaho Writers Guild

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    My solution to your problem has been to switch to Nosler Solid Base bullets, either Ballistic Tips or Accubonds and the Hornady BTSP, the one with the lead tip and crimping ring.

    All three have good terminal performance with the Hornady being a less explosive at ranges under 200 yds on deer size game.

    If cleaning reveals an extraordinary amout of copper fouling I wouldn't hesitate to run at least a partial Tubbs Final Finish process. I've use the Tubbs FF once on a new factory barrel with out testing the necessity of it first. All I can say is that it sure didn't hurt anything.
     
    Last edited: Nov 12, 2009
  9. Mikecr

    Mikecr Well-Known Member

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    +1 on the Tubbs FF
     
  10. 7man

    7man Active Member

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    Thank you all for the input.

    I think I know why I had that string of shots across the paper. For the 89 gr. load I never let the barrel cool between shots. I took 5 shots one right after the other. The barrel was hot enough where I couldn't hang on to it. I think this is an important factor in the stringy group and the yaw.

    I've been shooting at the range every night this week and I decided to go against what a lot of you have told me to do. I decided to not clean the gun at all. Now don't get me wrong, I like everthing you guys suggested, but I think I might have been cleaning the gun too much to begin with. I've been using Bore Tec bore cleaner after every shooting session. So every new session I start I'm basically starting over with a new barrel that is ripping up the bullets as they travel down the barrel. It was mentioned earlier that my barrel may be hard on the bullets. So, I figured I should let the imperfections in the rifling fill in as to create a smoother surface. I think this will work to certain point.

    Today after work I went to the range with a rifle that had 15 rounds through it uncleaned. I was able to achieve a .753 inch 3 shot group with this recipe.

    175 gr sierra gameking
    91 grains Retumbo
    Remington brass
    Federal 215 primer

    The difference today was I let the barrel have ample cool time between shots, and I had a fowled (seasoned) barrel. I'd shoot one round at my target with the 7 RUM, then shoot my .22 pistol to pass the time for cooling. Thought it seemed like a logical think to do.

    Now, what do you guys think about my theories here? Am I making sense, or am I just looney?
     
  11. 7man

    7man Active Member

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    The reason why I didn't switch powders and bullets yet is I have 100 sierra's and plenty of retumbo. Thought it would be worth spending the time with this combonation.
    I will be buying some 160 gr accubonds on the future though.

    Thank you
     
  12. royinidaho

    royinidaho Writers Guild

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    After that experience T'were it I, I'd definitely give the Tubbs FF a go....

    However, here is another fella's experience.

    There was a fella acting really down in the mouth at the Los Alamos range this past summer. He had a 30-06 Rem 721 that was passed down from his grand father, father to him. As far as he could recall it had never been cleaned. It was a great shooter he said.

    He got the idea that maybe he should clean it. He did and accuracy went wild, on the order of 3 MOA @ 100.

    The only thing that had changed was the cleaning.

    He came back the next week with 3 boxes of factory ammo. I think it was around 35 shots later that he had his accuracy back and resolved to never clean it again!

    FWIW
     
  13. elkaholic

    elkaholic Well-Known Member

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    I've had the same experience as Roy mentioned with a couple of old guns. Near tack driver with a dirty barrel to 2 moa with a clean one. I currently have a 6.5 Sherman with a Lilja barrel that I've learned not to clean down to bare steel as the groups go away and it takes quite a few shots to bring it back. This from a LAPPED barrel!.....Richlightbulb
     
  14. Mikecr

    Mikecr Well-Known Member

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    There are prefouling solutions.
    I keep my guns cleaner than brand new. It's just something my father impressed in me. "Never put away a dirty gun"
    What I've used for ~25yrs for prefouling is graphite, or lately(and better) WS2.
    Just a dry burnishing after cleaning the barrel.

    I've done this so that I can clean everytime, and protect the bore. The worst thing you can do to kill cold-bore accuracy, is leave oil in the bore. But you can't allow your bore to corrode either. The next worse thing for accuracy is generally the cold-clean-bore.
    So I eliminate both with cleaning AND prefouling.