Oops, the revolver was 7.5 in vs. 5 for the auto. Apples and oranges. Comparing a .45 auto rim fired from S&W 625 to an ACP fired from a Gold Cup, both fired from a 5" barrel. AR, 230 grain fmj, bullseye 5.4 grains at 800 fps, ACP 230 gr FMJ bullseye 4.6 grains at 800 fps. Using W231 with same bullet, AR 6.3 grains at 800 fps, ACP 5.4 grains at 800 fps.
It's almost all in the barrel length isn't it? "Revolver barrels are measured differently than semi-auto barrels. In a revolver, they just measure the barrel itself (and ignore the cylinder and chamber). In a semi-auto they measure from the barrel crown to the breech face. Measured the same way, a S&W 940 2" has the same barrel length as a 3 3/4" semi auto....is usually a little bit slower (a bit less than 5%) from the barrel cylinder gap."
So to really compare apples to apples we would need to compare revolvers with semi's that are 1 3/4" longer than the revolver wouldn't we.
It appears from your numbers that the cylinder gap loses the equivalent of about 2 inches of barrel length. I would call that significant.