Stainless Steel Barrel pitting

elkhuntr

elkhuntr

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Hi folks.

For interest stake and learning, here are some categories I have found around barrel erosion. All of which appear to manifest more as a barrel is shot.

1: Heat cracking and associated pitting. Variations from normal use, heat, fast firing say 17 shots in under 10min, or rapid firing without waiting.
2: Chemical pitting and damage. (rusting included, yes SS can oxidize just a lot slower or faster if oxygen and chemicals are in play)
3: Mechanical damage through cleaning or other means. (Is ammonia based cleaners really a no no for SS?)
4: Barrel metalurgy, micro pits and fractures manifesting as a barrel is shot.
5: To the extreme of a shot out barrel with bore enlarged and rifling removed for 2-3 inches in front of the chamber.

Now that I have a target barrel in the death throes of its useful target life I am wondering about your experiences with bore pitting. I would like to see your experiences, causes, dos and donts, and any thoughts on normal or excessive bore erosion?

Thanks in advance :cheers:
 
The debate whether SS material is better for barrels than Chromoly is still not concluded. While SS is indeed more resistant to the elements and fingerprint caused oxidation, the bore pitting, especially so near the throat region is still an object of debate. The main reason is the sulphur in most commonly used stainless steels for barrel making. Sulphur molecules are literally "blown off" the lattice by ablation from the burning powder particles. This lifting of the sulphur molecules leaves the throat pitted which now further accelerates the pitting process. So, in a literal sense, the throat and rifling is being "eaten" away due to sulphur presence. Now, there are low sulphur and, I believe, no sulphur SS alloys but I don't know how many by what manufacturer they are employed in barrel making.

For what's worth, for a barrel that I was planning of using extensively in target shooting and keep it as long as possible, I would go for carbon steel rather than SS unless I knew the latter was very low or no sulphur content.
 
Very interesting Miltiades. Indeed. keep em coming.

The pitting I have experienced is in the region 2-3" in front of the chamber. Otherwise, the bore and lands are in very good shape fore and aft with no pitting evident.
 
Thanks, by the time the bullet and burning powder have traveled some distance down the barrel, the latter has "cooled" enough to lack the energy required to displace the sulphur off the lattice and the barrel now behaves more or less as a carbon barrel would as far as pitting. On average, SS barrels tend to be harder that carbon so the rifling will probably stay "sharp" longer than on carbon steel but this in practice is a moot point since barrels get degraded in the throat area long before the rifling is gone.
 
elkhuntr said:
3: Mechanical damage through cleaning or other means.
Click to expand...

The dust from polishing media (particularly if treated with that red or green compound) can reduce barrel life a lot if you aren't blowing or swabbing it out of the cases before loading them. It can also wear your dies if not removed from the outside before sizing.
 
Miltiades said:
The debate whether SS material is better for barrels than Chromoly is still not concluded. While SS is indeed more resistant to the elements and fingerprint caused oxidation, the bore pitting, especially so near the throat region is still an object of debate. The main reason is the sulphur in most commonly used stainless steels for barrel making. Sulphur molecules are literally "blown off" the lattice by ablation from the burning powder particles. This lifting of the sulphur molecules leaves the throat pitted which now further accelerates the pitting process. So, in a literal sense, the throat and rifling is being "eaten" away due to sulphur presence. Now, there are low sulphur and, I believe, no sulphur SS alloys but I don't know how many by what manufacturer they are employed in barrel making.

For what's worth, for a barrel that I was planning of using extensively in target shooting and keep it as long as possible, I would go for carbon steel rather than SS unless I knew the latter was very low or no sulphur content.
Click to expand...

I see very few (if any) CM barrels in F-class and TR competition use.

99.9% use SS barrels, and probably for good reason.
 
Hmmmm, now that is something I can easily manage. I sure have caught the ones with red kernels locked in place. Always pays to inspect every piece of brass before loading. I will definitely be looking at swabbing out the residue. Would you replace the media more often or try to clean it some how to reduce the dust?

Thanks very much for this informative tip.....


I note that Kreiger mentioned CM can also make great barrels and is more of a preference thing for blueing relative to performance. Barrel life, now that is a debate upon itself. I have heard and saw a vid which indicated SS is easier to machine with less chatter so it can be chambered and contoured more easily. I wonder though who in the professional shooting world stick only to CM and why....a good discussion all the same.
 
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elkhuntr said:
Would you replace the media more often or try to clean it some how to reduce the dust?
Click to expand...

I use an ultrasonic unit, so I don't have to worry about abrasive particles form tumbling media in my bore and dies... But, as a final step in my case preparation, I always swab out each case out with a bore swab to get any dust or brass shavings as well as any case lube that may have found it's way into the neck.
 
Cyan1de:
I tend to aggree with you, but, sometimes "popularity" doesnt always mean better for an application.

One question I have is:

Does the process of manufacturing Moly steel produce a more 'pure' metal than stainless?
Or the opposite?

I know the smelting & alloying of stainless steels to be a very involved process.

Could the pureity of the two alloys be more of the issue?
Both steels are subject to the same wear and firing process. Some say stainless lasts longer, then you hear the converse.

I thought I would pose this question to see if any metalurgists are in the forums.

Lets see what us gunnuts have for information resources!
 
Stainless is 'tougher' to machine.
Speeds/feeds and tool sharpness/angles have to be good.

I have only. Built a few dozen barrels while working with a friend of mine, and this seems to be the case.

The moly machined beautifully. Easier to leave a fine finish when taking small cuts.

I have a small amount of machining experience due to my trade (millwright). But havent touched a lathe in 7-8 years now.

There must be some machinists in this crowd?
 
To keep things on track, the issue is pitting in SS barrels. Although I welcome very much the chatter aside:)

Yes...I am not a machinist and it is good to clear a machining fallacy up where it exists or is stated. What I do know is I have a 1000 rounds fired in a 300wm, with a hammer forged CM barrel, which still looks silky smooth except for the throat area as expected. It does not have the pitting in comparison to my SS barrel. Go figure....
 
Out of curiosity, do you shoot the same loads in Each rifle?
It would be very interesting if a given load would wear one type of barrel steel faster!

Sorry I will try to keep more on topic Elkhuntr.
 
Not the same in the cases I offer. Different calibers and diets. I wonder about the issue of cleaning regimin playing in here as well. Any comments out there on procedures that may cause issues in SS?
 
Damage due to cleaning regimen will cause scratches and surface roughness but pitting is the result of material ejected from the alloy, as with sulphur being removed from the alloy matrix. Now to the comment by Cyan1de as to the majority of the rifles in competition being SS, maybe their barrels are low sulphur. However, my opinion is that they are more worried about loss of accuracy due to throat erosion rather than pitting. The latter will not necessarily result in loss of accuracy while the former will. Furthermore, some buddies of mine who are serious competitors don't keep their barrels for very long and they replace them as soon as accuracy deteriorates. For them, perhaps the more corrosion resistant SS and the longer lasting rifling around the muzzle override any pitting concerns or effects. As far as I know no one has ever demonstrated any intrinsic accuracy superiority of SS over CM or the other way around.
 
In CM barrels the rifling tends to wash away from the throat forward and lose its edge and depth. SS barrels keep their rifling definition better, although they fire crack and pit from the throat forward.
 
CM barrels erode differently because of their lack of a "passavation layer", basically rather then pitting it will just rust away. Stainless steel erosion is actually called pitting erosion and is caused by surface defects which could come bullet accelarating to 3000 fps in .002 seconds.
 
clatartar said:
CM barrels erode differently because of their lack of a "passavation layer", basically rather then pitting it will just rust away. Stainless steel erosion is actually called pitting erosion and is caused by surface defects which could come bullet accelarating to 3000 fps in .002 seconds.
Click to expand...
We may say the same thing but using different terminology. CM barrel pitting is caused by "neglect" in the sense that is oxidation (rusting) which is a chemical reaction. The iron in CM steel simply binds to oxygen and the iron oxide now "exits" the alloy. SS can also pit due to oxidation at a much slower rate and the pitting, if it occurs, tends to be surface effect. However, the severe pitting of SS is a physical process not a chemical one. The fact that the displaced sulphur can oxidize after it has been ejected from the lattice in SS does not contribute to the primary mechanism of pitting. Kind of esoteric but interesting because it defines where pitting can occur in CM and SS barrels and what can be done, if anything, to minimize it.
 
I actualy saw that 260Rem AI barrel and can assure anyone that this particular one has many deep pittings 2"-3" in front of throat well beyond of sulfur depletion or just common "washout". I think that is relatively "rare" steel impurity that got passed by barrelmaker QC and got exposed after couple of hundred shots. I advised elkhunter to shoot it to his hart desire as long as there is sufficient hunting accuracy left in it, after that up to 3" chamber side barrel can be cut of (there is plenty of "meat"there) and brand new chamber reamed with excelent rifling condition past that bad spot.
 
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