You can also look at it terms of actual numbers.
Slugging the bore gives the handloader a fairly accurate account of actual bore size. Agreed or no?
Lets say you slug the bore of your 1911 45 acp and as with most pistols it measures .451. Jacketed bullet also measure .451. If the bore diameter is identical to the bullet diameter , would not a reasonable conclusion be that the bullet-to-bore fit is a tight fit and gas "Blow-by" is more of a theory then a fact.
Maybe it's just my thick head, but I can't grasp this.
A properly sized cast boolit will seal the bore and contain the pressure til the boolit leaves the barrel. Now a Jacketed bullet will not conform to the rifling enough to seal it, hence allowing gases to pass by it and not allow for complete potential pressure buildup.
If the jacketed bullet is already the same diameter as the bore, there is nothing to "Conform" to. It's already a tight fit. If anything the pressures on the base of a jacketed bullet should cause a somewhat tighter fit similar to the lead bullet, although not as much.
IMO, no way there is Gas-by-pass when using jacketed bullets. They are tight before the round is fired and they will get tighter after the round is fired.