pressure release in revolver vs autoloading pistol?

Its called a cylinder gap. On a good revolver, the gap will be about as wide as a piece of paper. There is some loss of energy there, but the same could be said for the operation of a gas operated semi. Now, to be fair most semi auto handguns are recoil operated, but most rifles are gas.

keep in mind that most revolver chamberings are inherently more powerful than a semi auto to begin with. (.357Mag, .41 Mag, .44mag, etc)

So, although there is a minimal loss of energy through the gap, its not of any real consequence.

Tahoeguy has it right.

Yes. There is a loss of energy. Fairly significant, and definitely measurable with a chronograph. However when compared to a semi-automatic, the same cartridge will result in about the same speeds because of the energy required to operate the semi.

A major difference, is the ability to suppress a semi-auto, while very few revolvers lend themselves to suppressors. The gap on a revolver lets enough gas pass by to negate any advantage of a suppressor. One notable exception are the surplus Nagant revolvers that overcome that issue by forcing the cylinder against the forcing cone as you fire it. Of course they also have a trigger pull best measured with a cattle scale, a pick up truck and a fence post.

A single shot, like a Thompson Contender will always give higher velocities with the same cartridge.

My concealed carry instructor was showing us how not to get our fingers in the way of the cylinder gap on a revolver: he folded a sheet of paper in half and tented it over the gun while he fired...confetti.

Energy visibly lost.

your describing gap difference, and yes there is a noticable amount lost but not as bad as semis ive found (i also just plain dont like semi autos though)

As soon as the round is fired energy is lost, thus creating trajectory of a bullet. As the round is fired, it travels in an arched pattern, not straight forward, kind of "falling in style." So, the velocity of a round is the greatest at the moment it is fired, anytime after that it is literally falling out of the sky. An autoloading pistol would be far more effiicent in terms of energy loss because it uses the gasses released by the previous round to cycle the next one, so on an so forth.