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bullet holes in target. (not perfectly round)

 
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Old 11-11-2009, 11:29 PM
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bullet holes in target. (not perfectly round)

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 .
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?
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Old 11-12-2009, 06:28 AM
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Re: bullet holes in target. (not perfectly round)

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
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Old 11-12-2009, 07:47 AM
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Re: bullet holes in target. (not perfectly round)

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.
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Old 11-12-2009, 08:18 AM
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Re: bullet holes in target. (not perfectly round)

"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
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Old 11-12-2009, 08:22 AM
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Re: bullet holes in target. (not perfectly round)

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.
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Old 11-12-2009, 08:45 AM
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Re: bullet holes in target. (not perfectly round)

Quote:
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.
Quote:
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
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
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Old 11-12-2009, 01:35 PM
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Re: bullet holes in target. (not perfectly round)

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?
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