Twofer,
If the bbl was originally fitted to the receiver, and done correctly, it is very highly unlikely that any work would be needed on the joint roll pin or the "hooks" on the bbl.
When a bbl is fitted properly, the breech face of the bbl should make as close to 100% contact with the face of the receiver. This should leave a very small gap in the water tables. This small gap during the fit, allows for the set back of material around the joint roll pin and the breech face.
When fitting a bbl I like leaving a .005" gap in the water table, near the receiver. After a few thousand rounds and opening and closing the gun just as many times, the metal in the above areas will have compressed, to close that gap to about .003". This should give the live of this fit, into hundreds of thousands of rounds.
There are a few things that can affect this longevity. Not keeping everything clean, and closing the gun with too much muscle. Closing any break-open firearms by slamming them shut, is akin to beating on the above areas with a hammer. This is likely the case on most guns that develop breech face problems.
Correcting a problem with a breech face gap can be as simple as replacing the joint roll pin/screws, ie..Beretta, Krieghoff, with larger diameter pins. Sometimes others may need a larger pin pressed into the reciever.
Welding the hooks can be a tricky situation. It best to have fixtures made ahead of the welding procedure, to ensure the correct location during the remachining. This can be somewhat costly. I have also silver solder round bushing into the hook areas, then remachined. This sometimes works out much better. less heat is needed, and some bbl just don't take to welding well. Also using a round bushing allows picking up the center of the hooks radius without too much guess work. Of course, having the correct tooling to do all of this is very essential.
Herb S.,
As long as the bbl's are locked tight, there's no problem. You can remove the bbl's, and take note at where the top lever sits with the bbl's. Compare with and without, and this will give you an indication on how much further it can go. The Miroku, like most Brownings, Winchesters, Perazzis, and such, all have a locking bolt that slide into a slotted lug area on the bbl., all can be corrected.
I should note that a lot of guns that have their top levers "wore" to the point that yours has, may develop a problem of "top lever jump". This is likely because the top lever springs may have never been replaced and have weakened.
A few guns like the Merkel, SKB, Weatherby and a few others, use a different locking system. This cross bolt locking system kind of has a "catch-22" when it comes to fitting. If you're wanting the top lever positioned anywhere before that 6:00 positition, the locking bolt will stick out of the left side of the receiver. This will lead to catching on your clothing or cutting your hand. So the end of the bolt will be fitted flush with the receiver. The top levers tend to be at the 6:00 position. As the bolt and bbl lugs wear, the bolt will continue into the receiver and the top lever will move further to the left.
The "catch-22" is that some guns do not allow the bolt to go any further into the receiver, it may be stopped by the top lever, or someplace in the bolt's location. This leaves the door open for the gun getting loose in a few thousand rounds.
I figure every manufacturer has their explaination on the proper fit of this locking system. I guess my option for this fit would be to have the top lever at the 5:00 position and the locking bolt flush with the left side, but alowing the bolt to continue to move as it wears deeper into the receiver, maybe another .025", and the top lever bottoming out at about 6:30.
Anyway, I've rambled on long enough.