There were two really big problems with revolving rifles back in the day.
The first was already mentioned. When you have a revolver, there is a gap between the cylinder and the barrel. If you did not have this gap, the cylinder would not be able to rotate between shots and you would not be able to open the cylinder to reload. Gas and bits of powder and small bits of metal come out of that gap at very high velocities. In a pistol, this is no big deal since your hands are behind the cylinder, but in a rifle, all that gas and all those small bits come out at high velocity and go directly into your left arm.
This video demonstrates the problem very well:
How NOT To Shoot a Revolver:
As mentioned above, there have been some revolvers, like the Model 1895 Nagant, that have some way of dealing with this. The Nagant moves the cylinder forward so that it makes a better seal when firing. Still, I’m not sure I would trust to put any part of my body next to the cylinder, just in case.
This video (by the same person, hickok45) demonstrates the difference between a regular revolver and the Nagant:
The second problem back in the day was a problem common to all revolvers, not just revolving rifles. This was the days of black powder, where you separately loaded a powder charge into each cylinder, then put a lead bullet on top of the powder and rammed it into place, and then put a percussion cap on the back of the cylinder (then repeat that 5 more times for the other chambers). If you did not keep the cylinder clean, any unburned powder on the end of the cylinder could cause a spark to jump over to the next chamber, causing it to fire, which is a really bad thing since that chamber is not in front of the barrel at the time (basically the cylinder ends up blowing up in your hand). Worst case, you get kind of a chain reaction where one chamber sets off the next all the way around the cylinder. Hence, this is commonly referred to as a “chain fire”.
These were such a problem that revolving rifles in the Civil War ended up being restricted to only using one chamber, which at that point you’re back to having a single shot rifle, which kinda defeats the purpose of a revolver.
The chain fire problem was solved with modern cartridge-style ammunition, so no more loose powder and no danger of one chamber setting off another. The cylinder gap problem can be solved using a Nagant-style cylinder or some other mechanical method that closes the gap when firing. The Nagant was slow to reload, but you could use a different system like a break-action that will eject all six spent cases at once and make the chambers very quick and easy to reload.
Yes. If I recall correctly, the Model 1895 Nagant’s forward-moving cylinder was designed more for better bullet velocity reasons than safety reasons. But either way, yes, you get significantly better bullet velocity if the chamber seals against the barrel.