Hi DavidTN! You're definitely on the right track! BUT!!! You have to find a caliper that is capable of .0000" (the last place can measure in .0005" and be close enough) to get an accurate grip on the issue... Also, as you have noticed, your breechblock is kinda of a "go" "no go" type of gauge all by itself.... Lay a straight edge along BOTH sides of the receiver, hold it up to the light, and find out which of the forks is the offending member... (They're certainly not bent equally!) Then shim and clamp the receiver in a smooth jaw vice and give the bent fork a bit of a flex... you'll have to exceed straight to get it to stay... for lack of a better term, sneak up on it!! Take your time, little bit, little bit!!! ...and if it all goes to hell in a handbasket, you never heard from me!!!
Best of luck to you, til...lat'r....GT