Is this... mildew in my upper receiver?

[gxer]

Came back from a visit to the range where I put 200 rounds of Blazer 124gr FMJ through my 16" CMMG RDB AR9. I know how dirty the rifle can get due to the blowback nature so I usually clean it right after the range time, today I get a surprise as I removed the bolt:

No doubt there's a lot of carbon fouling in there but on top of that, is that mildew I'm seeing? How can it form within a couple of hours after the range? I'm in CA where the air is dry, sun is bright, and there was no rain either. I'm sure those where not there at the range since the mildew looking substance is on top of the carbon fouling. Anyone have seen this before?

 

[KZP]

What brand lubricant are you using?

 

[gxer]

What brand lubricant are you using?

Ezzox

 

[2aBaC̶a̶]

strange. sure its not just some strange dust on top of carbon oil mixture?

 

[Steve_In_29]

Whatever it is I can guarantee you it is NOT "mildew".

 

[Sogan]

Ya idk what it is but I agree that its not mildew. Clean it off and carry on.

 

[roostercogburn98]

Looks like lint. What kind of bag/case do you transport it in

 

[hafejd30]

I get a similar bluish residue from moisture in mine. Mainly when using a can

 

[Evintos]

It's just oxidized residue from burnt propellant (bismuth, carbon, lead, tin, etc. - some will begin to crystalize). You see it a lot on muzzle devices with a lot of rounds through it without cleaning and the residue is left to oxidize. On muzzle devices they'll usually also have teal colored/green tint because of copper.

 

[Niles Coyote]

Oxidized copper trace

 

[acudaowner]

your gun has mold lol wake up one day and that mold will sprout fruit .

 

[clcustom1911]

Oxidized copper patina magic.

Clean? Wtf. No.

Lube and carry on until stoppages occur for proper break-in

Shoot the shit out of it.