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throat erosion?

 
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  #1  
Old 01-25-2004, 07:31 AM
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throat erosion?

what is it that causes throat erosion? velocity of bullet or large case of powder? I am currently working on loads for a 300 rum and was wondering if loading 220gr Sierra Match King bullets would extend the life of the barrel as compared to 190gr Sierra Match King.

does anyone know the rate of twist in a Sendero 300 RUM? would the 240gr Match King work in this rifle?
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Old 01-25-2004, 08:06 AM
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Re: throat erosion?

The twist on the factory Sendero barrels is 1:10. I've never shot the 240 gr SMK in mine.My bullet of choice is the 220 SMK and it gives great results. I can't say if it will cause more throat erosion then the 190 SMK though. I've always been under the impression that erosion is a function of heat and pressure. Seat the bullet as close as you can to the lands and keeping the barrel cool and clean will go a long way to keep erosion to a minimum.
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Old 01-25-2004, 09:57 AM
dwm dwm is offline
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Re: throat erosion?

I agree, I think throat errosion is caused by heat and pressure. So what causes extreme heat and pressure in the throat area?

I propose that it is heavy bullets with very long bearing surfaces, barrels with very fast twists, and relatively fast buring powder.

Heavy bullets take more force to overcome forward inertia.

Bullets with long bearing surfaces take much more force to cut the rifling in the jacket and experience more friction going down the barrel.

Tight twists take much more force to over come the rotational moment of inertia. (spin up)

These effects combine to cause the bullet to be held in the throat longer and the heat and pressure caused by (relatively fast) buring powder melt the throat.

So those heavy, really long, high ballistic coeffient bullets and the fast twists required to stablize them are the cause of rapid throat errosion.

It really bites that this combination is required for great performance at long range.

I think the only saving grace it that this situation can be compensated for somewhat with really slow burning powder and very long barrels.

Gain twist barrels may help somewhat by allowing the bullet to get out of the throat before it has to spin up.

Doug

[ 01-25-2004: Message edited by: dwm ]
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Old 01-25-2004, 03:05 PM
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Re: throat erosion?

Doug,
<BLOCKQUOTE><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><HR>So those heavy, really long, high ballistic coeffient bullets and the fast twists required to stablize them are the cause of rapid throat errosion.<HR></BLOCKQUOTE>

I'm going to take a somewhat different position on this one and say, there's probably more to do with shoulder angle, neck length, PSI, duration of burn (charge wt), powder burn rate and bore diameter than other things mentioned. A lot of it has been verified and is pretty well accepted, I still have reservations about some of it though.

We're talking throat errosion here, and charge weight and pressure while sand blasting the throat may account for much of this alone. The change in barrel time for a 180gr bullet verses a 240gr bullet is about 1.4 MS verses 1.55 MS (milliseconds) while peak pressure remains between .65 MS and .75 MS in each, don't matter what burn rate, within reason.

I'm guessing, but I would think that the additional dwell time for engraving is very, very minimal, if any longer at all...

The advantage not mentioned, but you burn much less powder too.

I'm fixin to go shoot some 178's with 103gr Retumbo, but the 240's use 10% less...

I'll have to add up the rounds I got through this barrel and measure the errosion again, I've got to have about 200-225 rounds through it so far. Insignificant the last time I measured it.
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Old 01-25-2004, 03:39 PM
dwm dwm is offline
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Re: throat erosion?

I'd like to see a test done with a .223 WSSM as an extreme case.

Build two, one with a slow twist say 1-14 and one with a fast twist, say 1-7.5. Now shoot 40 grainers in the 1-14 twist and 80 grainers in the 1-7.5 twist.

Use suitable powder for each bullet weight.

Which one will eat the throat first?

I could have said 6.5mm or 30 cal, but I think you get the point.

So where is the flame ball during the additional dwell time? Buring out the throat?

We are talking about very small time durations, yes.

How long does the powder burn and where is it when it is the hottest?

Something to think about ...

Doug
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Old 01-25-2004, 03:59 PM
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Re: throat erosion?

Doug,

That would be a very enlightening test. Not a bad idea for one of the gun writers who always need something new to write about anyway.

Another question while we're on the subject; which causes more throat erosion, large quantities of slow burning powder or smaller quantities of faster burning powder?

VH
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Old 01-25-2004, 04:04 PM
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Re: throat erosion?

I think alot has to do with the size of the powder grains, with the majority of powder still un-burnt at the throat its all hitting the barrel at high velocities causing a sand blasting effect, with the larger grains hitting with more force thus causing more errosion, As varmint hunter said light charges=less errosion it is because more powder is being burnt in the case and less hitting the throat. As for heavy bullets causing more wear, its probably because those people are using slow powder, and slow powder is usally larger grain. I think bullets have an almost zero effect on throat errosion, if bullets caused all the errosion, the barrel would wear at the middle/end, causing that part to go before the throat.

Brian
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