I'm no gunsmith. I spent a few years in a precision machine shop making optical equipment. If a hole was reamed to the same diameter as a hardened dowel pin, the pin pretty much could not be made to go into the hole without some trick, like chilling the dowel pin to shrink it.
If a hole was reamed .0005" over, a hardened dowel pin would press fit in part ways with a lot of force and was considered permanent, as in it could not be removed without damaging the pin or the hole. Getting perfect alignment between the axis of the hole and the axis of the pin was critical to fitting the pin into the hole.
If the hole was reamed .001" over, the hardened dowel pin could be press fit in relatively easily (tapped in with a brass hammer, etc.), though still a tight fit. With effort, such a pin could be tapped out if the hole was not blind.
For the pin to slide smoothly into the hole with little to no force, but with no discernable wiggle, the hole had to be reamed .002" over.
All of this depended on the quality of the dowel pin, i.e. the tolerances of its diameter, roundness, etc., and how carefully the hole was made. Plus, the bigger the pin and/or the deeper the pin was to go, the more room it needed, up to a certain point. This is just hands-on experience with holes 1/16" to 1/2" diameter, and 1/8" to 1" deep.
--shinbone
