He means colour the entire round in with a sharpie then try to chamber the round, where the marker rubs off that’s where your interference is. Another thing you can try is clipping the round under the extractor on the bolt before trying to chamber it. Mosins are a push feed and the extractors can be stiff and maybe part of your problem
OK, this is not the word of God, I am not even a Mosin enthusiast or gunsmith. I have one late 50s/early 60s import, so what I say applies to this one example.
I watched 5 dummie rounds feed into the chamber. As the bolt picked each one from the magazine, it pushed them along toward the chamber. Not one hooked under the extractor as you would see in a CRF Mauser. Of course not, it's a push feed.
When the bolt hit the top of the action, that is the handle straight up vertical, I could pull the bolt back and the round stayed forward, no extractor engagement.
At this point, with all 5 rounds, I started turning the bolt down a few more degrees each time, listening and then pulling it back out to see if the extractor had engaged the cartridge rim. On this rifle the cartridge gets grabbed when the bolt handle is pointing straight out from the right hand (45 degree) action flat, or at 1:30 if the clock face is used. The bolt on this rifle is fully closed when the handle is at 3:00, or straight sideways, horizontal.
When I pull the bolt part open, I can just barely get the extractor to lift, about 1/32", maybe 1/16", but it is quite stiff. Following Webley No.5's lead, I suggest you soak the extractor some more and work it with a piece of wood until it has good movement. Have fun.