This is one of those questions where it's hard to answer without looking at a particular slide & barrel combination.
I certainly don't recommend someone try to 'fit' a barrel without having the knowledge and experience in doing so properly, but I can explain a little of what's taught to armorers of 3rd gen guns, just for educational purposes ...
When installing a new barrel, armorers are told to initially 'seat' the barrel by installing it in the slide and then tapping on the bottom of the feed ramp block with a plastic mallet. (FWIW, in the Sig armorer class we were told that a P220 .45 barrel might require 'seating' with a brass mallet under some circumstances.)
Then, the slide would be turned over (so the sights were pointing 'up'), and the freedom of movement of the barrel would be checked to make sure the rear of the barrel would easily (under its own weight) drop down freely from the 'in-battery' position, using the tip of a thumb to lightly lift and drop the rear of the barrel a few times. If the barrel does not drop freely the barrel tab would be filed until the reinstalled barrel did drop downward from the 'in battery' position under its own weight. (File too much, ruin the barrel.)
Once the barrel passed the first bench check the gun would be reassembled and a feeler gauge of .007" would be inserted between the breech face and barrel tab when the pistol was slightly out-of-battery, and then the slide released to let it try to close into battery by pressure of the recoil spring. The slide should remain slightly out-of-battery when released with that gauge in place. If the slide is in battery and the .007 gauge can be pulled out easily, the tab has been overfiled. Barrel should be replaced. (This feeler gauge check is only for fitting a new barrel, not for inspection during maintenance, since the barrel tab will 'wear in' from use.
Naturally, the revision making the barrel tab much wider (and changing the breech face machining from having 'rounded' shoulders to hold the case base to having a 'straight' vertical cut) needs to be considered when having to replace a barrel, since this production change doesn't allow for mixing slides & barrels of the different designs.
Now, I've only come across a few S&W pistols which required some adjustment of the barrel tab.
The first couple of them were easily dressed just by filing the rear of the tab (although those particular models required a different gauge dimension according to the tech with whom I spoke in the factory, and which I used after discussing it with him).
Another one involved some minimal filing of the rear tab, as well as the edge of the tab shoulders. It didn't take much in order for the barrel to pass the bench check where the barrel easily slid into place and then dropped under its own weight, and the feeler gauge test was appropriate. Then, once I was confident with the armorer bench check results I proceeded to a live-fire test and confirmed normal feeding & functioning with different ammunition, too.
Now, as time has passed S&W has started to run out of stock of some of the earlier 3rd gen barrels. Last time I asked them they said they had a limited number of early 4566 barrels remaining available, for example. Also, when the CHP started requiring some new barrels for their older 4006's, the factory didn't have any and had to modify a few 4006TSW barrels by hand in order to make them fit the slides of the older guns. (I was told the TSW barrels have thicker barrel hoods than the older 3rd gen models and the entire barrel hoods had to be reduced in thickness by hand, which is a bit of a laborious process.)
I've never had to adjust or fit a barrel lug or muzzle, but then that issue has not been addressed in any of the 4 armorer classes I've attended over the years for 3rd gen guns. If I were to encounter such a problem I'd either keep trying different barrels in a slide, to find one suitable for that slide with no (or little) fitting being required, or else return the gun to the factory to let them resolve it.
