Dave,
With the brass shims that are thicker than the loose areas, you get an interference fit between the parts. This means now the square of the tumbler is larger (with the shims) than the loosened "no longer square hole" in the **** and has to be forced into place. It won't be a perfect flat edge to flat edge fit in a worn **** hole, but it fills in some of the loose area where the shims aren't actually shaved down by forcing them into place.
You are correct that brass shims don't last quite as long as steel ones, but they do last a long time when they are oversize and are a forced fit. The "bit of give or ability to shave/scrape down in the tighter spots" of brass shims can be and often is a benefit in a forced fit. You don't run quite the risk with the original parts of the tumbler square or **** cracking with them. This because these parts were normally casehardened Iron and not a tougher steel alloy.
Of course with either a brass or steel shim, if you have to take the **** off the tumbler for some reason later on, you really should use fresh shim stock for a new press forced fit.
This discussion has reminded me of one incident on the second time we went to the Colonial Williamsburg event "Under the Red Coat." Our Unit decided to do a different event, though anyone who wanted to do the CW event was invited to fall in with the Grenadier Company of the 42nd. When one brand new member asked me which event I was going to attend, I told him I was going back to CW. He had wanted to do that event as well, so we met there.
A couple days before the event I got an urgent call from him asking if I was going to bring my Armorer's tools. I said I was and then he told me the **** was off his lock and he could not get the lock out of the musket. I told him to STOP trying anything and just bring the gun and parts to me in the parking lot and meet me an hour sooner than we had planned on meeting.
Fast forward to the parking lot at CW and he handed me the musket, **** and tumbler screw. I asked him how the **** had come off and he said, "Oh, when I did Civil War reenacting, we pried the hammers off our locks to clean them all the time in our unit. So I did the same thing with my Bess." I replied, "Good Grief, what ***** started that nonsense? Do you remember if the **** was on full ****, half **** or all the way down when you pried it off?" He said he didn't remember. So I loosened and tapped gently on the screws and the lock would not budge. So I got out an adjustable wrench and wrapped tape around the jaws, tightened them down on the tumbler square. Then I tried moving the tumbler and realized it had been on full ****. So I pulled the trigger with one hand while holding onto the tumbler and wrench with the other and let the tumbler move gently to half ****. Then the lock tapped easily out of the stock.
I explained that the **** HAD to fit tightly to tumbler and that was the LAST time he had best ever pry off the **** from the tumbler or he would face the full fury of the Armourer's Wrath!! I also told him whoever taught him to pry the hammer off his Civil War Enfield Rifle was woefully ignorant or stupid and he had best not do THAT again, as well. I disassembled the lock and he had worn the hole in the **** a bit loose from prying it off. I told him I would peen it for the weekend, but after the event, I would take his lock home and shim it. I also taught him about putting the **** on half **** to take the lock out of or back in the musket. I had him do that a few times on our free time in camp to practice while I supervised him.
Gus