Help from M1A1 Stock experts !!

[painter777]

Think it's really burn marks or Soot build up? Soot is a heavy Carbon build up.

If a bore cleaner like Hoppes#9 won't do it.

Mix a cup of Chlorine bleach, 2 cups of water and add 5 or 6 tablespoons of TSP ( Tri Sodium Phosphate ) mixed together. TSP is a common cleaner found at hardwares. Stiff bristle brush should do.
Might take several applications but normally this should do it. It's a common cleaner for fireplaces.
There may be special Fireplace cleaners out there, I just haven't looked.
We've used this mix in bigger ratios for fire jobs to clean up Fireplace mantels, beams, built ins...etc. Works very well. Usual precautions for bleach. TSP is more or less harmless. Eye protection...and gloves are the main thing.
Just keep the outside dry as possible.

I've heard Hydrogen Peroxide or oven cleaners work, but have never tried them.

If no luck....As a last resort...
Use a 2 part Wood Bleach.
Be careful and contain it to just inside.
If it gets outside it will pull color from the exterior of the stock.

Good Luck,
Charlie-painter777

P.S. Try searching fireplace cleaners. There may be some new mix out there I'm not familiar with.