Strange Phenomenon

[psteinmayer]

I'm experiencing something with my Toyo Kogyo Series 31 that I'm have a hard time coming up with an answer for it. Something of a head scratcher!!!

My son shoots this rifle in CMP matches, and this has happened twice now: Some of the cases have a burnt appearance on the cases past the necks. It's as if the brass itself is burnt. Here's the thing though... it isn't every case... just some of them. The first time I noticed it was when I was decapping the cases before I threw them into the tumbler. There were seven or eight cases that showed this. some of my brass is Norma, and some of it is Privi... but this has showed up on both, so I know it's not the brass.

So, at the last match, I loaded different brass to take the cases out of the equation. There were two this time: The first was the third shot, and the blown back material was mostly carbon and relatively minor. The second one was the fifth shot of Off Hand (30th shot overall) when the barrel was somewhat hot, and the case was pretty blackened back down one side over half the length of the case. The next five cases came out looking just fine! I tumbled this case, and although the case cleaned up, the burned appearance seems to be permanent! I will attach a picture later if needed.

There has been no issues with extraction/ejection, and the rifle shows no signs of damage. I viewed the chamber as best I can and I see no issues. The rifle fires just fine and the accuracy is as it should be! I don't think it's unsafe. My reloads are relatively mild (Hornady 174 gr FMJ, 40.0 grains of IMR-4064, CCI-200 standard primer). I'm just curious to figure our what could be causing this strange phenomenon...