I thought this might be a worth while post for the Mosin lovers like me on this forum. This is not necessarily a C&R post so I chose to put it here.
I believe that the better people understand the different features and mechanics of something like this the less they will be afraid to make quality improvements on their own rifles. I am going to try to go step by step on each part of the bolts travel and what you are feeling at each step and how to improve it if so desired.
Starting with the bolt fully rearward and the magazine loaded. As you start to push the bolt forward the first thing the bolt will touch is the rim of the cartridge case. In my rifles this is almost no resistance at all.
Once the bolt is all the way forward, but the bolt handle is still vertical, there is a notch in the bolt body and cocking piece that is holding the bolt handle upright. In order for you to bring the handle down to finish closing the bolt, force must be applied to push the handle down and rotate the bolt body enough to snap out of this notch. You can easily feel it if you bring the bolt down a bit and back up. For some rifles this can be quite a bit of force. I have seen people fail to cycle the bolt because the force required was surprisingly high. I polished the notches on my scout earlier today, but I ended up actually removing part of the notch to reduce the force required to snap out of it. All I did is polish it down a bit so it was not as sharp and snappy.
I used Superlube synthetic grease on the polished up ramps and they feel like glass sliding on glass. Much better!
It is easy to see the notches in these two pictures. They mate and can be quite stubborn to defeat.
Once you have defeated the notch with the bolt rotating downward there is a ramp on the bolt body that pushes the bolt forward into the breech while allowing the locking lugs to do their job. This rides against part of the receiver. The smoother this is the easier this part of the bolt cycle is. I polish this ramp.
There is another opportunity for resistance as this part of the cycle is happening. The extractor must slip over the case rim as the rim snaps into the bolt face recess as the bolt is pushed forward into the breech. The two things that cause resistance here is the extractor sliding over the rim and the rim catching or dragging on the sides of the bolt face. To help alleviate this I polish the part of the extractor that slides against the case rim and I very slightly chamfer and polish the inside for the bolt face recess.
One more source for resistance in this cycle that will effect the feel of bolt is the underside of the connector. This is the bar that connects the cocking piece to the bolt. The machining on this thing is horrible in my rifle! I used 180, 400, and 600 grit with oil to smooth up most of the roughness on this part. It is not necessary to remove all the tooling marks. Just until it moves easily. This is not required but I think it cleaned up the feel of my bolt quite a bit.
Once the round is fired starts a whole new part to the bolt cycle. This bolt cocks on opening. There is a lot of force involved in cocking that firing pin so the bolt has a lot of resistance to lifting. This is where the radiused ramps come into play. They provide the mechanical leverage to push the cocking piece back against the firing pin pressure. I used 400 and 600 grit with oil to smooth these up before polishing. I was careful not to change the surfaces much at all. Just smoother.
Once the bolt is nearly vertical it has to overcome the notch that holds the handle upright again. This is another part of the bolt cycle that a smaller notch could make things faster and easier.
I ignored the possibility of a sticking case for the purposes of this post.
I really think that most people who have issues with stiff bolts might benefit from understanding how these parts of the bolt cycle effect the feel of the rifle. Once these things are accounted for, fixing issues like sticking cases can more easily be diagnosed and fixed properly. I have some work to do on my wife's scout Mosin. Writing this all down will help me work through each stage and get things right without chasing ghosts.
I hope this was a worthwhile read.
Nice. Thanks for the write up.