"I can't understand why this hasen't, at least to my knowledge, been addressed and solved,"
I can; it really isn't a problem. The people who design, make and sell revolvers are not and never have been fools. The Nagant revolver system certainly "sealed" the gap at signficant cost but for no measurable ballistic effect. (
Really good drawing hickstick!)
You must presume the gas pressure escapes from the cylinder gap with a force equal to what it applies to the base of the bullet. If so, that isn't so!
Use your physics to consider the density of the gas and it's speed down the bore. The gas is like a high velocity slug of thick syrup and little of it escapes through that gap. A 30kpsi chamber pressure makes the gas some 2,000 times more dense than air and the inertia of that very high density resists a 90 degree change of direction when it leaves the cylinder, so very little of the pressure or gas actually escapes.
Or you could put an "O" ring in the gap.
