I have seen a number of five groove barrels in which the throat is defective, due to reamer deflection. In these barrels, there is evidence that the reamer has cut in the grooves but has not fully removed the lands. This is the result of one cutting edge of the reamer being unsupported by the others. The reamer essentially bounces back and forth as it cuts. This phenomena is made worse if the reamer is a bit dull. As I said earlier, this is most common when the throat is cut separately but it does happen with reamers which have an integral throater as well. The deflection is not huge, and to the naked eye, the throat appears normal. The amount of deflection is probably on the order of .001" or slightly less but it is still enough to leave enough land to contact the bullet. If one has the ability, it is possible to detect the deflection in the neck as well. In a throat which has this defect, the functional diameter of the throat will be somewhere near nominal bullet diameter or just a little less.
Is it correctable? Yes, it is. The use of a spiral fluted throater, or a five fluted throater, will allow one to remove the remaining tops of the lands. The throat will still be a little weird because the grooves will but radiused differently than they should be, but the throat will work out OK.
I have to emphasize, with the right tooling, the right set up, and the right technique, this will not happen. It is my opinion though, Ruger has an issue in one or more of these areas; at least in this chambering. GD
