Sand, or "golden" color powder granule residue indicates incomplete combustion. It is not exactly unburnt powder, it is under burnt powder. The outside coating (with the graphite giving the powder its gray/black color) has been burnt off, but the rest of the powder granule did not ignite, leaving it a yellowish color.
Those granules are in your chamber, not because they were sucked back from the barrel, but because they merely stayed about where they were when the case was extracted.
Think of the old trick with the tablecloth. The gun is the table, the case is the tablecloth and the powder residue is the dishes. Yank the case out fast enough and the "dishes" stay right where they were, on the table (in your chamber)_
Incomplete combustion usually doesn't happen in small cases with medium burn rate powders, But in this case, obviously, it did. I cannot tell you exactly why, I only have a couple of guesses that MIGHT or might not be accurate.
First one, obviously, is defective primers. (or insufficient primers). Enough flash to ignite some but not all of the powder.
Second guess is lack of pressure. Most modern powders are "progressive" meaning they are engineered to provide a constant burn rate as they burn and the pressure increases. Some of them will not burn completely without a high enough "backpressure" on the powder charge as it burns.
In either of these cases, then CAN be enough pressure to push the bullet out of the barrel and still have incomplete ignition of the entire powder charge.
Really strange to see golden sand in a .357 Sig with AA#7 and a charge that is right in the middle of the range listed in my books.
I did experience this incomplete ignition once in a rifle. I got a couple pounds of really slow magnum rifle powder in a trade deal, and didn't have any of the big belted mags to use it in. Eventually I found a load listing it in the .45-70, and I loaded up 3 rounds to test with. Marlin 1895 lever gun. First shot, (it wasn't a heavy load,) and when I ejected the case, I saw the case come out trailing a "golden shower" of grit. The bullet had gone downrange, but I swear it seemed like half the powder charge was golden sand that dumped out of the case on its way out of the rifle and afterwards. I did fire the other two rounds, and the results were the same. And after those three shots the actions went "crunch" when you worked the lever. Complete tear down and detail cleaning took care of that, and I never tried that powder again. I'd have to go look at it to see the exact name it was 8700 or something like that. Still on the shelf in my storage closet, if someone wants to make me an offer....?
you've used those primers and that powder before in other loads and all was good, yes??
You're sure nothing contaminated the primers or the powder? yes?
This one is a head scratcher to me.
Something here isn't right. Bad primer, bad power, bad load, seems like its got to be one (or all) of those, but I don't see how that could be.
POSSIBLE solutions MIGHT BE change primers, change the powder lot, or change the load density. What ever you do, use caution. This is absolutely an off normal situation, what should be a normal combination is not giving normal results.
You're off the map here, mate, and there are dragons out there. some you can see, and others you can't until they eat you. Tread carefully...
Good Luck.
