Jerry,
In general, a properly chambered rifle will require no polishing for function but the polishing gives the surface a more uniform appearance. I polish using a piece of the finest Scotch Brite I can get. The barrel is turning in the lathe, the Scotch Brite in a slotted mandrel in the old Black&Decker. Actual elapsed time of the polishing operation is likely about 10 seconds. Total time is considerably more than this, of course, since it includes remembering where I put the mandrel. While looking for the mandrel, I'm likely to notice a magazine lying on the bench and pause to read an article of interest. This is best done with a cup of coffee in my hand so I have to go get that. On my way to fill the coffee cup, I'll trip over the dog so I'll kick her out the door. Once the door is open I'll have a look around and it will occur to me to get a chunk of wood for the fire. When I go out to the wood shed, I'll find the chainsaw file I'd misplaced. While I have it in hand, I'll file the saw then cut a few pieces to test it out. When I finally get back into the shop, I'll look at barrel in the lathe and remember I was going to polish the chamber. Going over to the bench, I'll notice a magazine which contains an article I've been meaning to read...... When I do get around to polishing the chamber, I don't think there is any measurable change in dimension. Over the years, I have refined my technique and now that I'm about ready to quit, I think I do OK.
A lubricated chamber undoubtedly increases bolt thrust. However, it is the extent to which it does so which is important. In the case of the primer protruding 30/30, bolt thrust went from zero (the head of the case didn't contact the bolt face at all), to what it would have been if there had been no head clearance in the first place. In fact, the unlubed case probably did stretch backward a significant amount but not enough to exceed the elastic limit of the brass. In other words, the brass was able to return to it's original length or very nearly so.
A front locking bolt action rifle probably deflects less than .001" at maximum pressures. It takes little force to stretch the brass case that amout so the lubricated case will increase effective bolt thrust only by the amount required to deflect the brass an amount equal to the deflection of the bolt/ receiver. I made up a split chamber once so I could clamp a case in it. I set up a dial guage on the chambered piece to indicate deflection of the brass as I pushed agaist the inside of the case with a rod. I could move the case head by .002" with a pretty hard push. Maybe 200 pounds of force. 200 pounds is a drop in the bucket considering the amount of total thrust at 60,000 psi pressure. Regards, Bill
