Hello all. I am running into a new to me problem with a Finnishicon M39

Here are the pertinent details:

Mosin Nagant Model 39
Bore Slugs to 0.313
Bullets: Hornady 174 GN RN Diameter 0.312
Brass: Graf and Sons
Powder, Reloader 15 46.6 Gn (Box max per Hornaday)
Book Velocity 2600 fps
Measured Velocity 2200-2300 fps average

All cartridges come out of chamber with powder residue and there is powder residue on bolt.

Also there is a bad smell after firing (like sulphur)

It seems to be that pressures must be low and eratic (hence low and variable velocity). Since the brass is coming out dirty and I have that smell issue, it seems like the cases are not expanding to the case walls in firing to properly seal the breech.

I think the throat (at least) is toast.

Any different ideas or confirmation of my theory?

Thanks
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