"The .357 and .430 bullets don't fit into the barrels of my 686 and 629, so the barrel diameter is obviously less, but I can't measure it accurately without a plug gauge. I guess this goes back to my first question. What is the factory shooting for on bore diameter and how much tolerance is allowed in manufacture?"
More information for you. Most people measure the full bore diameter by doing what is called "slugging the barrel". A soft piece of lead such as an egg shaped fishing sinker is driven through the barrel and measured for diameter at the largest point. This process will give you an idea of what diameter cast bullet to use in your gun. You usually use a cast bullet that is .001"-.002" larger that the slugged diamter. If you're interested I think I have a link to some information regarding the process.
Cary