The constriction at the end of each cylinder bore is called the "throat". If the throat is smaller than the barrel you are going to have accuracy issues and at times Ruger has shipped guns in that state. We also have the issue of pre-2007 large-frame Rugers having throats each cut with a different bit/reamer set (all six going at once) so you can have variances between chambers...not good as far as accuracy goes.
To really do chamber reaming right, push a round lead slug through your barrel and measure it, and then set your throats to fractionally over it.
Since I did a completely custom barrel and cylinder for other reasons (in 9mmPara) I set my throats (which in an auto caliber are very long, damn near an inch!) to .3555" to be fractionally over the .355" barrel. I then buy .356 lead hardcast bullets for best accuracy, or at least I would if I could score pistol powder
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