Why are my brass so dirty?

[woodnut]

I have shot ARs with adjustable gas blocks and suppressed. Until now, most brass comes out only slightly dirtier than a bolt rifles brass. Well, maybe a bit more than "slightly",ok. But , my AR10 in 260 shucks the dirtiest brass I ever saw! What is going on and how do I change it?
I believe I have the gas dialed down to just operate, but do I? And is that all that's causing it?

 

[MudRunner2005]

Probably your powder choice... What powder are you using for your reloads? Some burn dirtier than others. For example, I found H110 burns much cleaner than whatever Hornady uses for factory .357 Mag and .44 Mag LeverEVolution ammo.

Or, the chamber has loose tolerances around the neck, and it's not sealing properly against the chamber walls, when the charge is ignited before the bullet exits the barrel.

 

[woodnut]

Thanks Mudrunner, but the ammo was factory! I've shot this AR lots and no brass came out this dirty. I'm still guessing the adj. gas block isn't set correct. More tests!

 

[ar10ar15man]

low pressure is a poor burn and dirty.
suppressor does not help

 

[Earnhardt]

Rifle gas?
Early unlock, allowing gas back and onto the brass?

 

[woodnut]

Ah ha! Earnhardt, that makes sense! I'll get roundtoit and do some fiddlin with the gas block.

 

[BallisticsGuy]

Dialing your gas system down one notch at a time and finding out if it's down far enough is the wrong way. Start with it closed and slowly open it until the gun just starts cycling. NOTE: For target shooting purposes, the occasional failure to cycle is better than an early unlock.

260 ammo is typically intended for a bolt gun which means the powder choice was almost certainly not optimized for a gas gun.

 

[Earnhardt]

Ah ha! Earnhardt, that makes sense! I'll get roundtoit and do some fiddlin with the gas block.

Maybe try a little heavier buffer, and like these guys are saying, start with the adj block closed and slowly open it up.
I had the exact same problem you are having, with my 45 Raptor.
An extra heavy buffer, with an adj block fixed it

 

[Varmint Hunter]

I've got a RRA in .223 which shoots well and ejects fairly clean brass. But ..... when I put a RRA 458 SOCOM upper on the gun it spits out the dirtiest brass I've ever seen. Soot like residue over most of the case. It easily transfers to your fingers.
I've used RE7, H110, Lil'Gun and H4198. They all seem to cause the same dirty cases. I am at, or near, top velocity so I should also be near max acceptable pressure too.