Tried the acetone on a small brass wire brush. Didn't even phase it. And I have no doubt that carbon and / or copper will build up inside a barrel. I always go thru a lengthy cleaning routine at the end of each range session. I'm sure there's carbon in the throat area, probably a little fire cracking & who knows what else.

This seems to be none of the aforementioned. Brushes & chemicals I have on hand have no impact on it. The inside of that barrel was traumatized with a brush earlier today & it made no difference at all, even scrubbed with Sweets & Hoppes.
Nutthin'.....

I give up.

I'm going to shoot it once more. If it gets worse & more of that whitish hazy stuff appears, I'll be doing an early barrel change out. So far the trend has been the haziness has increased with each range session, over the past 2 weeks.
Thanks for the input gents.... I'm off to the 'ol salt mine.

Frank in FLa