Weird defect in my old BR barrel!

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xs hedspace

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Noticed a shadow in my 6mm 14 twist Wiseman-McMillan BR barrel, took my borescope to it. Holy crap! There's a pitting that looks like little worm tracks about 20" from the breech! Anybody ever seen this before? The rest of the bore is lovely, but I haven't shot this thing in 6-7 years, and a group on a windy day was less than 1/2" with Berger 68 gr on Monday, so I guess I'll just shoot the thing in a 300 yd F-class match this Sat, and hope for the best....Can't figure what could cause this, covers about 30% around the bore, zig zagging around about 1/2" long.
 
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You did not say ...

whether this is a stainless barrel or not, but I'll assume it is, and that corrosion is not the cause. In which case, it is likely an inclusion in the steel itself. Such defects are not rare, but more often are found in the form of a 'pipe', or slag inclusion in the center of the barrel, resulting from slag in the original billet, which was then rolled into a bar, leaving the pipe along the center of the blank. Often this is removed in the actions of drilling and reaming, and thus never detected at all, but sometimes is large enough to be seen afterward. I have drilled, contoured, reamed, rifled, and lapped a barrel blank (a .338!) and only then detected a pipe the full length of the bore: while it might have been reamed-up and re-rifled, it got pitched anyway. Smaller inclusions might be covered by only the thinnest shell of material in a finished barrel, and only later become exposed by flaking-away of the steel. It is not likely a safety issue, but, if the defect is large enough, might cause mechanical damage to the bullet, serve as a focus for metal fouling, and have a bad effect on accuracy.
mhb - Mike

xs hedspace said:
Noticed a shadow in my 6mm 14 twist Wiseman-McMillan BR barrel, took my borescope to it. Holy crap! There's a pitting that looks like little worm tracks about 20" from the breech! Anybody ever seen this before? The rest of the bore is lovely, but I haven't shot this thing in 6-7 years, and a group on a windy day was less than 1/2" with Berger 68 gr on Monday, so I guess I'll just shoot the thing in a 300 yd F-class match this Sat, and hope for the best....Can't figure what could cause this, covers about 30% around the bore, zig zagging around about 1/2" long.
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mhb said:
Such defects are not rare, but more often are found in the form of a 'pipe', or slag inclusion in the center of the barrel, resulting from slag in the original billet, which was then rolled into a bar, leaving the pipe along the center of the blank. Mike
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Could you elaborate Mike? Especially the phrase "......which was then rolled into a bar, leaving the pipe along the center of the blank."

I'm having trouble picturing how it's rolled into a bar.... I've never even driven through a town with a foundry so far's I know :)

Thanks!
al
 
In the interest of perfect honesty...

I've never actually seen the process, either.
But I have read about steel making, and worked in barrel making with a metallurgical engineer - who explained the process to me.
As I understand it, all commercial steels these days are cast in large billets - there is an effort to remove the slag from the melt before the casting, but sometimes a certain amount of slag is retained in the melt. In the casting process, the billet cools from the outside to the center, and any retained slag tends to migrate to the center of the casting, which part solidifies last. The casting is also often trimmed to further eliminate retained slag, but, again, not all of it is always removed.
When the cast billet is reduced to round bars, such as that prepared for barrel making, the original casting is reduced to the desired shape and dimension by the processes of extrusion or passing through successively smaller rollers - in either case, any slag which was originally in the center of the casting is retained in the center of the final bar stock, resulting in the 'pipe' phenomenon. Also, any residual slag is most likely to be found in only one end of the original billet, so that, of the full amount of bar produced from any given billet, only a portion will likely be affected by the retained slag, if present.
Finally, quality control in the steel making process should, and usually does, eliminate any stock which may contain such inclusion, but (obviously), some occasionally slips by.
mhb - Mike

alinwa said:
Could you elaborate Mike? Especially the phrase "......which was then rolled into a bar, leaving the pipe along the center of the blank."

I'm having trouble picturing how it's rolled into a bar.... I've never even driven through a town with a foundry so far's I know :)

Thanks!
al
Click to expand...
 
Fascinating! Thank you :) I had no idea..... My thoughts on "slag" originate from casting lead, the slag's always on the surface, I couldn't picture it ever getting into the mix. Especially not "rolling into the center."

Now I'm picturing 70-80ft long hunks of hammered out barstock with one end possibly loaded with a "pipe" of impurities.... and these getting trimmed down to around 60ft and sent around the world???

Hey, I still don't know much but thanks to you I know more than I did....

al
 
Yes, it's an old SS barrel, and the inclusion idea makes sense. Since then, I borescoped the rest of the barrel, and found two more areas of "worm tracks", about 12" apart. Damn, and I just paid Larry Racine to rechamber this barrel for me. I think it's time for a new tube, since the 300 yd F class match scores went from mediocre to doggie doo by the third 15 shot string, last Sat. Thanks for the info.
 
I had one of those inclusions reveal itself in a LV barrel at less than 200 rounds.

This barrel would tease with a really good group then three or four average groups.

Ed Shilen borescoped it, found the problem and called it an "Ugly Spot"......I got a new LV barrel.
 
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