[BJK]

All long shots but here goes...

With no scale to work with I have to ask. Can it be unburned flake powder?

Or were you shooting lead bullets previously? Could it be lead or coating coming off the bore?

 

[BJK]

I strongly doubt that it's the firearm itself. But to rest easy, clean the barrel and peer down it. My bet is that it will be shiny like a mirror with the rifling easily seen. If it was from the rifle itself where else could it come from but the bore? You'd also see other issues if the bore was coming apart or flaking in some way.

I still think it's powder. Who knows what they were using for powder when that stuff was loaded? Not so long ago everyone was seeing shortages and who knows what they used? Many years ago (50ish?) I saw similar flakes coming from my 9mm reloads out of a handgun but I can't definitely remember the precise circumstances. That was many years ago before I had the means to quantify anything and I flew by the seat of my pants. In my case they might have been compressed loads using PB powder (long out of production) when I was foolish enough to try to "magnumize" the 9mm.

I recently read data for the 300BLK where adding more powder resulted in lower velocities. Clearly the powder wasn't being ignited. My bet is that something put on the ground in front of the bench would see the exact same flakes as what you have.

Dred gave you a good idea. Get the heavy bolt weight from KT. It won't screw anything up at all even with light loads and should give more time for full combustion. I have one in mine and put thousands of butterfly fart loads through it and if anything would produce malfunctions with that heavy weight in place my competition loads would have done so. Nothing to lose except a few $ and everything to gain. To you the shooter you won't notice any significant difference in the operation of the rifle, maybe a hair less recoil.