Strange Borescope Image

While we are talking borescopes and bore cleaning, thought I would share these photos. The rifle is a No. 4 Mk 2 7.62 DCRA Conversion. 1960s Long Branch 4 groove. I noticed this strange pattern in the grooves of the rifling the first time I borescoped it.  Lands are perfectly smooth, no such marks.

I'll show the image to the metallurgy guy at work, but my best guess is corrosion

Here is one of the worst areas.

This is a good example of why routine and effective barrel cleaning is important. 

Yes, it has to be that.  

Thanks Paul, seems there is good consensus it is removed corrosion. Likely the JB that removed it, or past times when I’ve brushed the bore.  It just looks odd under magnification. 

Its pitting corrosion alright, typical pattern I see all the time on steel where there is corrosion under insulation on pipework.  Will have formed from residual oxides in the forging process when the barrels were made.  The buildup of fouling over the decades has to an extent stopped it being much worse than it could have been.  More modern barrel forging methods tend to prevent this nowdays.

Does not affect accuracy, this is my most accurate No. 4.  My Dad had the rifle sent to Long Branch thru DCRA for the conversion. When he gave me the rifle, he didn’t think he had ever fired it, but in searching his shooting and reloading records, I found he had shot 10 rounds of Canadian Service Ball ammo thru it (mid 1960’s). Not very satisfied with how it shot (poor ammo, but he didn’t know that back then), he never fired it again.  He would have cleaned the barrel afterwards, but that would not have removed any copper which is particularly bad for the first few dozen rounds on a new barrel, and this is where the fouling would be heaviest (where the pitting is).