Thanks for the replies.
The barrel gap is not my problem, it is cylinder pin binding around the bushing. As you can see below the Uberti 1875 does not have a very large bushing and the pin itself is quite small in diameter. Even with Mobil One it starts to seize up fairly quickly.
In another test I swabbed the inside of the cylinder pin hole with Mobil One as well as the pin and the front of the cylinder. After 35 rounds it was time to clean it all. It didn't seize but it was definitely getting tight. So far the Mobil One keeps my guns running the longest though I'd love to get another 10 or 20 rounds out of them.
Willy
I find that it's not the lube on the base pin, or inside the bushing that's important. The trick seems to be to get lots of grease on the face of the bushing (i.e. between the bushing and the frame of the revolver) in an effort to keep the fouling from migrating down into the base pin.
I've noticed that, when my pistols bind up, only one small grain of powder has gotten past my "barrier" down into the base pin itself.
I have a sense that the more carefully I "seal" up the base pin to frame juncture, the longer the pistol will run. Usually, it goes the whole match without any problems. However, without "sealing" the juncture, the gun will get sticky after three or four stages.
Again, I use Mobil 1 for this purpose.