I recently purchased a nice, used Italian 20 gauge O/U for my wife to use that has chromed bores. After her "get acquainted" session at the local Sporting Clays range, we came home and cleaned our guns as per usual.....both of us noticed an unusual occurrence that I have never before encountered: as the bristle brush (copper) was pushed/pulled through the bore, small "puffs" of a whitish powder like substance were emitted from the muzzle and breech respectively. Looking down the bores, you could see striations that corresponded to the scrubbing pattern of the brush going through the bore......sort of a dusty or powdery appearance. Pushing a patch through left a silvery/brown residue on the cloth. Wasn't powder residue and wasn't plastic as we had already run solvent/brush down the bore and the bores were "clean"..........excepting the forcing cone area.....that's why several more passes were made with a dry copper brush and we noticed the silvery dust.

Anyone seen this before? Might it possibly be that this chrome plating is not up to spec and is eroding when we give the gun a good scrubbing?
(Not worried about it, gun has a permanent home 'cause the wife loves it.)
Just curious, because I've not encountered anything like this in my other shotguns with chromed bores.
Appreciate anyone's input or opinions.

I have several chrome bore guns and have not seen that. I do not think the white powder is related to the chrome.
I bought a new Beretta SP1 this summer. I had a lot of trouble getting the bores clean for the first 10 or so shooting sessions. In the extra long forcing cone area mostly. But whatever was in it finally came out and it is slick and fairly easy to clean now. I do not go for perfect right now as I am cleaning the gun each time on the truck tailgate because I do not want to do it in the campground.

Being a used gun, it's likely the previous owner didn't clean it well and left some leading in the bore. Over time, lead will form a whitish looking powder similar to what you see on car battery terminals. The chrome bore will protect the steel from damage. Just scrub the heck out of it with your bronze bore brush and WD-40 and enjoy your new gun.

Oh, yeah....I gave it the "once over" (complete tear down/pulled extractors/springs/firing pins, etc). Worked over the bores/polished chambers. (Bores are mirror bright) It's as clean as it will ever be.
The Missus' first outing with it produced 79 % hits on the course. Happy camper, she is!

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Definitely sounds like oxidized lead. Dangerous stuff to breath. You do not want cloud puffs any where near . . . well anything, especially, not children nor food. You need to consider cleaning or filtering your work area.

I get that there are old timers that have been breathing oxidized lead their whole lives without ill effect but it remains a very hazardous substance particularly to children.

Duly noted! Thanks!
I guess I've always cleaned my guns so, thoroughly, that I had not encountered this phenomenon before. Lends credence to Riflemeister's comment that the previous owner may not have cleaned this shotgun as judiciously as most of us do.
Good thing to be aware of when purchasing used..........
Thanks all!

I guess it could actually be lead built up in the bore. I bought some target shells at the range yesterday and cut one open to see what was what. The shells have a little plastic cup that comes up about 3/8 of an inch and the lead is just pored in with no sleeve or full length wad. so it is just like the old days with the lead rubbing the bore. I bet a lot of game loads are built that way too.

I use a copper brush on cleaning rod dipped in Hoppes # 9 , Put in a drill , and slowly push in and out, Do 3.4 times and bores are like mirrors Brings out stuff I didn't know was in there. Sounds like you had a lot of old reside in there.