We all know that using a short cartridge in a cylinder meant for a longer version of the same cartridge (like .38 Special in .357 Magnum cylinder) is no problem. In an ideal situation where "fit is king" and accuracy gains are vital for a competition gun, we would like to eliminate the freeflight where the bullet is unsupported before it reaches the chamber throat and steers toward the forcing cone.
Another part is that in order to achieve a great trigger job, the lighter the cylinder is the less weight there is to rotate in the double action pull. This is also true for a speed shooting gun where you want a light cylinder so that it can rotate faster, hence the titanium cylinder on new guns like S&W 986 and 929.
I've got a S&W 625 in .45 ACP. I've been using it with .45 GAP with great success. The advantage is a shorter cartridge that is easier to do fast reloads with in speed shooting competitions. The disadvantage is the long free flight of the bullet. With the new revolver rules coming up in IPSC competition I'm thinking about fitting a new barrel and shortening the cylinder. The longer internal barrel length would increase velocity, making me able to reduce the charge somewhat for lower recoil with the same velocity output as with the old barrel.
So to the main question. How would the bullet behave in terms of internal ballistics without a chamber throat?
I know there are methods to sleeve the chambers for a new throat, but I would rather avoid it due to costs and pain in time and labor. Also, a throat would probably mean the cylinder would have to be longer than in my sketch below.
(click on it to view larger version)