Strange Phenomenon

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...

Must be fairly low pressure reloads....lets gas back around the case 'cause the case isn't completely sealing the chamber. Normal phenomenon

I agree with Stephan . . . .

I think your load is "right on the cusp" of being too low of pressure . . . .

I suggest you bump-up the powder charge a mite to ensure complete obturation of the case.

I bet some of the brass is a tad harder/different than some others!

Bump up about ONE-HALF grain (1/2 grain, or .5 TENTHS) and I'll wager the problem goes away! --Jim

Madsenshooter also suggested the too-low-pressure cause when we discussed this (when it happened the first time). The curious nature of my brain said to see if it happened again (which it did twice on Saturday). Interestingly enough, this load is about middle-of-the-road in my Hornady 9th Edition, and I've been using this load for several years... but this is the first time I've encountered the phenomenon. I'll bump the charge up slightly and see what happens.

Work-hardened brass and tough military brass can be a cause the sooty case issue too. Try some fresh brass first if you are otherwise satisfied with your particular load.

Can also put a bit more crimp on the bullet...sometimes will help(I guess it pressures up more pushing the bullet out?)....I've had .30-06 150gr plinking loads that needed a good crimp or otherwise were likely to soot-up the cases...

Switch to something good, we all know them Jap rifles is junk anyways...lol

I agree, sounds like there isn't enough pressure. Do you know if it is in the same spot inside the chamber each time or do you never see it until the brass is out? Could be that the chamber is ever so slightly out of round and then hard brass+low pressure+out of round chamber= just enough for this to happen once in a while.

The first time... I didn't know about it until I was decapping the brass to clean. This time, we watched for it, but didn't know until after the brass was ejected. Brass has been annealed, and has a half dozen or so loadings through them. I believe you are all right about the low pressure. I'm going to bump it up slightly and see at the next match if it's still occurring.

Thanks for all of the awesome advice so far...
Paul

My circa 1975 Lyman manual shows the starting load for the 7.7 with IMR 4064 @ 42.0gr. with 180gr S.P.C.L. bullet (#B22812) giving 2,227fps. Their max charge of 4064 with the same bullet is 46.0gr @ 2512fps. With Norma cases and Remington 9.5 primers. My hunch too would be that your 40gr charge is on the border line of being able to consistently produce enough pressure to effectively seal.