That cylinder gap doesn't waste all that much energy, at least not one in industry specs....but it does bleed off a bit. How much isn't easy to measue, but from tests run setting the gap at differnt settings (and the one I remember reading didn't use a Dan Wesson...although that would work) it's pretty minor...call it no more than 3%.
Bottom line: The differnce from .000 to .003" is less than the natural variation between different barrels.
Have been some semi-autos (mostly in the lower powdered rounds) that allowed the slide to be locked in place to prevent semi-auto cycling. Have had two old Savage .22rifles that allowed locking the bolt...and just like the pistols, if there is any difference it's less than the normal vel. variation of the ammo.
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I too had a 2 3/4" Ruger Speed Six in 9mm and a Browning HP out on the same day. Back then, light weight bullets at high velcoity seemed worth exploration and was 'graphing some BlueDot/90gr.JHP loads. The difference between the Ruger and Browning semi-auto was less than 10fps (they both hung in there at 1400-1410fps averages).
Did notice one odd effect.
Shooting side-by-side into water jugs, the revolver's bullets tended to fragment more than the Semi-auto's loads. Same loads, same bullets, same velocity...but the revolver's rifling was deeper, and the bullet had a long distance to go to find engravement. Best guess is that that little bullet, moving along at a very fast clip, smacking into the deep rifling, stressed that jacket more than the semi-auto did...which allowed what should have been identical loads to sjow that fragmentation when shot from the revovler.
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BTW: Bullets can be moving pretty fast by the end of the cylinder. Guess it was illegal (would be a smooth bore pistol) but enough years have passed..fired a Dan Wesson without any barrel attached over a chronograph....some of the 158gr. loadings were doing a bit better than 800fps with a barrel length of "zero" (most were in the 640-700fps range, but a few brands were evidently loaded hot-and fast). That's a pretty impressive velocity...consider standing back 10 or 15 feet and shooting at a vice mounted barrel with a .38special and the stress applied if you were to shoot one dead center of that mounted barrel.