throat erosion?

Dr. John

Well-Known Member
Apr 11, 2002
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?
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.
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.


[ 01-25-2004: Message edited by: dwm ]
<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.
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 ...


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?

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, I think you missed my point. I didn't say the bullets were causing wear on the barrel. I said that the bullets were harder to get moving because:

1. they are heavy for the caliber and therefore harder to get moving forward (probably minimal effect, but still a contributor),
2. have a large bearing surface which is harder to push into the lands and down the barrel, doesn't the bullet deform as it cuts into the rifling,
3. the fast twist also makes it harder for them to get started moving because they are harder to spin up (heavy weight also makes this worse) thet have to move forward and spin at the same time while cutting grooves in the sides,

All these things contribute to the bullet (accelerating) moving out of the throat slower as the powder burns in the throat area.

This causes the flame ball to sit near the throat longer and melt the lands at the throat.

I was not talking about wear, I was talking about the flame ball melting the barrel.

Brent, don't almost all modern powders have a significant graphite content? Wouldn't this reduce the sand blasting effect?

I'm not saying your wrong, maybe it is the sandblasting.

I was trying to get everyone to think in terms of melting the barrel in the throat area. Then the powder and gas flow push the molten barrel metal out of the throat and down the barrel.

Maybe the sandblasting has the advantage of a very hot throat because of all the reasons I stated above.

Ever had an overbore case shoot the throat out of a gun ("early") that has a slow twist and is used only with light for the caliber bullets?

More to think about.


[ 01-25-2004: Message edited by: dwm ]

your right i did miss that last post, I think I can some up everything, With a heavy bullet more thoat errosion happens because the powder burns longer at the throat causing the barrel to become hot and soft and when you throw in the unburned powder hitting the soft metal at high speeds it causes errosion, I dont think graphite would help with the powder hitting the barrels walls, just reduces the friction of the bullet and reduces the time that flame ball stays at the throat.


[ 01-25-2004: Message edited by: Brian Rybicky ]
how can you tell you have throat erosion. is it obvious looking into the barrel? does it deteriorating accuracy? I have never seen it or experienced it.

I have always believed that long/heavy bullets had the potential to increase throat erosion for the reasons that you stated. However, long/heavy bullets also require you to use a lighter powder charge than a medium or light bullet in the same cartridge. Lighter charges almost always equate to less throat erosion.

I haven't got a clue as to which has a greater effect on the rate of erosion; medium weight bullets w/heavy charges OR heavy bullets w/medium charges.

I shoot em' all and rebarrel when necessary. I have to admit, the "Longrange Hunting" boys sure have me shooting many more of the heavy weights than I ever did before. I use to be one of those light, fast and furious kind of guys.


[ 01-25-2004: Message edited by: Varmint Hunter ]
Dr. John, most accurate loads fire best when loaded a specific distance off of the lands. This can be measured in differant ways, but as they "erode" away, most precision shooters chase them by loading thier cartriges longer. Some can tell by accuracy becoming degraded. Or check with special tools. Stoney Point makes one. I don't know how good it is, I have never used one. A simple one can be made by using a fire-formed casing in the rifle that fired it. Seat the bullet long, but not too tight (Remember, no powder or primer!). Chamber it, then remove it and measure the length. If the chamber is unusealy long, you will be able to tell. Some rifle manufactures just plain have very long chambers. Its what I call "Factory Throat Erosion." There is more too it than what I have listed, but I think you will get the gist of it. Hope this helps!
There is every bullet/twist combination possible out there in STD 308s.
I've never heard of a 308 burning out this side of several thousand rounds.
If I remember correctly, the 6PPC also offers a long barrel life.
No matter what you do with them, they will last longer than a 240gibbs or a 30-378.
And no matter you do with a 30-378, it will not last as long as a std 308.
Much as I hate it, the erosion definitely follows the amount of powder set loose in a given bore area. Pressure, boattail, powder type, dwell,etc, must all have minor roles, as neither, or all, cannot contradict powder AMOUNT away.
In otherwords, a 56grcap Win308 at 60kpsi pressure load will still outlast a 133grcap 30-378Wby. Even if it's loaded to only a 40kpsi pressure load.
I don't understand why. Simple energy amount, I guess.
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