Tight tubes at the block/FSB are a good thing as long as it is aligned properly with the port. Hard to measure for sure but the hole in the FSB is much larger.
As far as the fit of the other end into the key, it should not bind. It needs to "float" in the hole in the upper receiver. They will wear in, then last forever as long as they are not abused.
The fit of the key to the carrier could allow for leakage. I used a bit of Permatex Form a Gasket on a friend's build 15 years ago as he had a poor fit. This stuff is an automotive product that is solvent resistant, well, everything but alcohol. It worked perfectly.