Dave, that extractor shelf looks suspicious to me.  The angle looks like it will make the leverage even worse.  When assembled and the breech block open, does the extractor extend to back against the Breech block, with pressure applied forward to it?  If it doesn’t, I would for-certain agree with the welding.  With too much weld….or too high of a corner, it will probably lock up the breech block and not let it open or at least get tight.  Then it would be just removing metal until everything works.  Of course, I would bet you already know all this.
Bob

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If this is the case, mark the case like jack suggests, before firing. After firing look for any shiny areas on the case, fire it several more times and keep looking.

If a shiny area appears you either have a slight bulge or a bad chamber. If that's the case, you can open the chamber, behind that area, until it extracts freely.

My gut reaction, as alluded to above, is that the fault doesn't entirely lie with the extractor, rather with the chamber or soft brass or, heaven forfend, load hotness. Just a thought.

I'm diagnosing this real time, posting as I try things.  Hopefully this will be somewhat interesting and show how a person can go about sequentially eliminating variables as they trouble shoot.

What I've really come to realize is that memory and what we perceive are very influential on how we go about diagnosis, and perception and memory aren't entirely accurate.

I pulled the barrel.  The grease and crud in the extractor cut shows that the extractor isn't hitting and binding there.  The grease is from me greasing the barrel with bullet lube after cleaning.   

I tried fired cases in the barrel.   They chamber and extract readily, no sticking.  I tapped them in to refusal with a small hammer.  Tighter, but don't stick.  These same lot of fired cases jams nearly every time when loaded into the assembled rifle, and have to be pounded out with a cleaning rod, or by hitting the lever hard to get the extractor to work.

I then used pin gauges to check the chamber.  It is .001" over brass size at the back, and tapers evenly as evidenced by sequentially further penetration as I go down .001" between pin gauge diameters.  The brass also shows no evidence of bulging, and tapers uniformly when measured with a caliper, and isn't out of round.  So, pretty much have ruled out a bulged chamber.      

What that now says to me is that the extractor is the component causing the case to bind.  Looking at it and comparing to the chamber, the inside edge is slightly out of round.  I now hypothesize that it is binding the case.  I previously had determined that the extractor was binding slightly on the receiver wall, and relieved the back top of it.  When I check the back of it with a straight edge, it is now bowed slightly, so the extractor could still be binding based on hitting the receiver wall below.   

I'm now going to re-assemble the rifle, and check for sticking cases WITHOUT the extractor in place.  I should have done that before I disassembled it.  If they don't stick, I'll trouble shoot extractor more.

Now a note about perception and memory, as mentioned above.  This rifle was unused for 6 years.  Last year, we used it again, and cases stuck.  Seemed to be worse than before.  Maybe, maybe not.  I also am relying on memory about new cases vs sized, and fired.  And, I had previously polished the chamber out slightly to make it fit the cases better, as it was .001" tight at the back.  All those things, and time, confuse memory and affect how I perceive the problem.

Could be no more than the chamber needs a good cleaning/polishing.

Going to do something else for awhile, will take this back up tonight or tomorrow.  I put the barrel back in, and the breech block will press in the brass till it gets tight and needs knocked out with a rod.  Couldn't find any new brass, but sized a case and it doesn't stick.  I'm going to try one more thing, trim a fired case to make sure it's not the mouth.

Thanks, John, appreciate your insight.  I'll pull the barrel again, and will chamber cast it.  Really don't think it's a ring; it's pressing the brass in the last few thousandths in the chamber that sticks it.  It finger seats probably within .005" of final.  The breech block will cam it in to stick, tapping it in with a hammer won't.  Anything up to that last few thousandths doesn't stick.  Cerrosafe will give me assurance that what I'm observing is right.

Then, I'll polish the chamber again, and particularly the back of the chamber.  I'll pay particular attention to not bulging or egg shaping the chamber in the process.   

This chamber has always been tight and has had clearance problems since the beginning.  I pulled the barrel and polished out the chamber slightly when we first used it, and it made it better with new brass, but the problem remained with fired brass after it was reloaded and not sized.   The pin gauges show it's adequate for the brass, but a bit more taper would probably improve things.

lots of good long distance trouble shooting thoughts

here's my $.02 worth

1) resolve chamber question
   pick your favorite ...  for a tight chamber mine would be 
   dyekem blue or magic maker and see if a "perfect" chamber 
   reamer cleans up with a very light hand touch
2) build up extractor leg contact point after chamber / ammo 
    issue is resolved  ... see attached pic ... they look almost 
    straight yours looks like the point is very rounded

good luck