Why has S&W gone to a .427" chamber throats in this revolver? How can it POSSIBLY shoot cast bullets of .430-.431" after swaging them to .427"? I would assume chamber pressures are substantially higher as well. Of course I'm not a S&W engineer, just using common sense. I suppose I shall open the throats to .430-.431".

Did you measure the throats by slugging them & measuring the slug, or did you try to measure them by using a dial caliper directly in the throats? The most accurate way is by slugging. Trying to measure an inside dimension of a cylindrical object with a dial caliper will result in measurements several thousandths smaller than the actual dimension, for several reasons.

Having said that, although throats that small would definitely raise pressures, the real issue is if the throat is smaller than the barrels groove dimension. Did you slug the barrel, or are you asuming that .430-.431 is correct from what you have read? Slug the barrel, or have someone who knows how if you don't, to determine the actual groove diameter. A special V-block will be needed to accurately measure the 5-groove slug. Once this is done, and the throats slugged you will have the information to know what needs be done. Until then you are just guessing. As any machinist will tell you, measuring dimensions of the magnitude of .001" increments isn't so simple as it would seem, and errors are easily made.

Understood Alk; my concern is a .430" XTP would require a 2# hammer to push through the throats.

Well, that answers part of the question, the throats are less than .430 apparently. I say that because, even though it doesn't seem right, a .430 bullet will not go through a .430 throat freely. Even a .431 throat will sometimes offer some resistance since both the throat & bullet are usually out-of-round to some degree. The good news is that jacketed bullets will most likely shoot well even if slightly undersized, whether by design or being forced through a tight throat. Have you tried .429 bullets? Even if not casting there is somewhat of a selection in diameter of jacketed bullets in some calibers. .44 happens to be one of them. At least try a .429 bullet, but measure it to be sure of its diameter. The listed diameter is a "nominal" figure and can vary as much as .001" due to manufacturing tolerances. At least then you will have a base line to work from rather than guessing as it would seem you are doing now since you still do not mention that you have measured anything.

Alk; Throats measured .427" after slugging. Bought a Manson reamer to open throats to .431" as revolver is intended to shoot cast, medium to heavy bullets.