gatnerd said:

It would also just be odd to use steel cases here, as the sort of soft 'cartridge steel' used in Russian style steel case ammo is not appreciably stronger or that much lighter then brass...

If they are dummy rounds, strength and weight do not matter.

And there is precedent for using steel case dummy cartridges.

WWII, Korea Dummy Training Rounds M1 Garand, Clip, 30-06

The leftmost round has the lacquer chipped away on the edge of the case shoulder. It’s shiny in the shadow and at a different angle to the reflection of the lights on the cases. The cases are not polymer. The exposed metal is silver not brass or white. I’m interpreting it as lacquered steel.

A rather interesting reloading component has showed up; American Reloading often has obscure / military components (for example, the only website I've ever seen offering M80A1 projectiles.)

Now they have .277 Furry / 6.8NGSW cases. Quite expensive at $1 a piece as well; whether thats due to scarcity or due to them being expensive to manufacture, I don't know.

https://americanreloading.com/277-fury/3249-277-fury-sig-primed-hybrid-case-100ct.html

I'm going to email them and ask whether they can weigh one of the cases for us. 

Don't forget the cost of the primer and putting it in the case.

Buy them if you can .i would if i could 

I think we all got excited for naught here.    

Seems to be that SIG sells brass cased ammo with a black coating on it already on the standard commercial market. Should have checked because that kind of casing color wackiness is up SIG's alley. But at least it's more likely that those were Low Pressure 6.8NGSW cartridges, so all brass, that SIG has decided to apply its coating to for XYZ reasons. Over being a new generation of high strength steel cases.

The coating looks to be mainly applied to their 'Short Barrel 300 AAC' line. But I couldn't find any pics showing good amounts of wear to confirm that they also expose white looking material underneath. As the above linked cartridges seem to have white colored wear patterns.

Excellent find man. That seems to settle it. 

Apsyda said:

Seems to be that SIG sells brass cased ammo with a black coating on it already on the standard commercial market. Should have checked because that kind of casing color wackiness is up SIG's alley. But at least it's more likely that those were Low Pressure 6.8NGSW cartridges, so all brass, that SIG has decided to apply its coating to for XYZ reasons.

I'd say it's more likely that those were dummy 6.8 NGSW cartridges, that SIG decided to apply its black coating to for easy identification as dummy rounds.

Apsyda said:

The coating looks to be mainly applied to their 'Short Barrel 300 AAC' line.

Yup.  Black cases for .300 AAC Blackout cartridges.  It's kinda poetic.

Apsyda said:

But I couldn't find any pics showing good amounts of wear to confirm that they also expose white looking material underneath.

To my eye, the black case in this photo looks to be a good match.

Testing Sig's .300 Blackout Short Barrel Loads :: Guns.com

Apsyda said:

The coating looks to be mainly applied to their 'Short Barrel 300 AAC' line.

The coating was a request from some "end-users" to easily make the difference between subsonic and supersonic loads (shiny brass = supersonic; black = subsonic).

EmericD said:

       Apsyda said: The coating looks to be mainly applied to their 'Short Barrel 300 AAC' line.

The coating was a request from some "end-users" to easily make the difference between subsonic and supersonic loads (shiny brass = supersonic; black = subsonic).

SIG has gone beyond that original use.  SIG commercial .300 BLK ammo uses black cases on both supersonic and subsonic "short barrel" loads.