What is this stuff lining the barrel

[Wow]

About 200 rounds down the barrel.

https://imgur.com/a/MnWe4Is

[Arik]

Copper fouling from the copper bullet jacket. Normal. When you clean it the patches may come out a little green looking

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[Thunderpigeon]

You are seeing copper fouling. It can be removed with a copper remover such as Sweet's.

[Deadman William]

You are seeing copper fouling. It can be removed with a copper remover such as Sweet's.

keeping in mind it's pretty hard and time consuming to remove, and likely has no impact on your guns performance

[DG23]

keeping in mind it's pretty hard and time consuming to remove, and likely has no impact on your guns performance

He will get to a point where he reaches copper equilibrium (blasting it out almost as fast as it deposits).

Sometimes that can be an accuracy 'sweet spot' that is consistent and lasts for quite a few rounds before a deep cleaning is needed.

[T2C]

If accuracy hasn't suffered, I would not worry about copper fouling.

[markm]

What kind of barrel? Chrome lined? Stainless Steel?

FWIW, Sweets sucks at copper removal. I will, every so often, knock down the copper fouling in my Bolt gun 308. I used to use Sweets... then one time I followed up with Bore Tech copper remover, and there was a mess of green nasty left in the bore that sweets had left behind.

[Wow]

The barrel is a Faxon pencil profile nitrided barrel.

[pointblank4445]

Unless your barrel has taken a sudden performance dump, put the borescope away and don't fret on what you see.