Two comments, don't know if they will help though.
1-you have to remember the PPSH41 barrel is chrome lined, that might cause problems using a reamer?
2- I dug out my unmodded PPSH41 bolt and inserted a spent case, the bolt surrounds the base of the case up to the front of the extraction groove, in the pic this is about where the red line is. Click on the pic for a larger image
I can't compair it to the barrel because it's not here right now, but I'll do what I can when it is.
Now I'm guessing but this gap might close a little more when the heavy bolt slams against the round pushing it further into the chamber some.