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26-03-2018, 09:32 #1
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243 Accuracy Issues - Impact of carbon buildup
Good morning,
We spent the whole of Saturday trying to work up a load for a 243 with medium results.
The rifle just came back from gunsmith in Pta who gave it a thumbs up from a barrel perspective (which I have my doubts). The rifle was pillar bedded and fitted with a silencer. It is a Sako Finnbear with a 1:10 twist and a 586mm barrel.
The shooting was done on a friend’s farm who has a 700m range which saved time by driving to a fro from range. The following components were used:
Case: New Norma Brass
Primer: CCI 200
Powder: S355
Bullets: 90gr Nosler BT
The first load was 40.8gr which gave av speed of 2960fps which gave a very good grouping. The temp was 18degrees. Next load was 44.2gr which gave 3300fps with poor grouping. All the info was fed into QL as we received new data. We settled on 41.2 gr which gave a 3 shot clover group with all the holes cutting on another (ave speed 3000 fps, 26 degress). And that is when the problem started. The 41.2 load gave great grouping but after about 15 shots it would open up. We cleaned the rifle with Boretech carbon cleaner and the groups shrunk to initial group. After 15 shots the same would happen. The 15 shots were not fired consecutively and ample time was left for barrel to cool down.
After cleaning the barrel, it looked like the barrel has some visual small pits and the surface was not smooth compared to three other new barrel rifles. The rifling was also not crisp and they looked round as oppose to nice square rifling.
Has anyone else experienced a similar situation that a rifle will only group for a number of shots and then open up?.
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26-03-2018, 09:39 #2
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Re: 243 Accuracy Issues - Impact of carbon buildup
Who told you 44.2gr S355 was safe to use?
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26-03-2018, 09:48 #3
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Re: 243 Accuracy Issues - Impact of carbon buildup
Is he not using a 70gr bullets data?
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26-03-2018, 10:12 #4
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26-03-2018, 10:22 #5
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26-03-2018, 10:28 #6
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26-03-2018, 15:11 #7
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Re: 243 Accuracy Issues - Impact of carbon buildup
back to the original question. Can barrel fowling happen so quickly to affect accuracy?
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26-03-2018, 15:40 #8
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Re: 243 Accuracy Issues - Impact of carbon buildup
As far as Ive heard. 243 barrel life is not as long as other calibers.
Ive also heard. But maybe people more in know can say.
That apparently as you fire and powder starts burning.
Some powder travels down barrel and still ignites.
The powder leaving case that has not yet ignited hits the insides of the barrel at very high speed.
Almost like sandblasting it over time. And this causes barrel to get small marks just in front of chamber.
And causing rifling to deteriorate.
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26-03-2018, 16:13 #9
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Re: 243 Accuracy Issues - Impact of carbon buildup
Hi 264WinMag, from your description it looks like the rough and pitted bore might be the cause, although its difficult to tell without looking at it. A rough bore will scrape off and deposit jacket material, causing a build up or laminate of carbon and copper. Carbon cleaner does not remove copper, you will need a dedicated copper remover like Boretec Cu+2, MPro7 or in severe cases Sweets 762.
Bear in mind that this is only a temporary fix, as you will still be left with a rough albeit clean bore. A more permanent solution might be to get the bore lapped or live with constant bore cleaning.
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26-03-2018, 16:44 #10
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Re: 243 Accuracy Issues - Impact of carbon buildup
I would say no, carbon will not do that within 15 shots.
I think you guys witnessed some symptoms and associated the fix with cleaning without a complete documentation of events.
Anyways, if you guys did shoot at 3300ft/s you would have put enormous strain in the action, my first thought would be bedding, not barrel.
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