I wrote the following in a post about cerrosafe vs. swaged lead throat slugs, and it's still there, but decided to make it a seperate post, because it is so important and should be considered by anyone who makes a throat slug. The following will give a precision measurement of runout, however a very simple and fairly effective way to see runout of the bore relitive to the throat, is to stand a throat slug vertical and turn it slowly, to see if the rifling marks start higher up on one side than on the other. If they do, the bore is excentric to the throat and considerations must be made when reloading, with lead bullets especially. There are two short accounts below the following.
Nothing is better for checking the concintricity of a chamber/throat to the rifling and bore. Make the cast to include at least 1 1/2 inches of rifling and only a little throat is needed, but all the throat and part of the chamber is ok. Chuck the rifled portion in a lathe and use a dial indicator to be sure that portion is running dead true, or mark it's exact high and low spot, and amount of wobble the chuck is throwing into it. Then move your indicator to the throat/lead just before the rifling, (if the throat has a gentle taper) or with chamberings like the 45-70, 30-30 and similarly chambered rifles which have very abrupt chambers, set your indicator on the chamber neck area. If runout is more than .001, a bullet that is over groove diameter will be forced out of balance when fired, so make your bullets hard as possible and size them close as possible to groove diameter. This arangement almost always gives excellent accuracy, but as speeds are raised to the accuracy limit, expect some fliers. Not wild fliers, but group ruiners.
If chamber/throat runout is under .001, best accuracy will normally be obtained if that portion of the bullet held inside the case neck, and as far as possible into the throat, are made as large as the chamber will tolerate. The bullet body forward of that which extends out into the throat and to touch the rifling lead, should be cut close as possible to groove diameter, with as much as a half thousandth under groove being as good as an exact fit, or in most cases up to .oo1 or .002 over groove being just fine. The object of fitting this way, and why it gives the best accuracy possible is that a full chamber neck area will not allow the bullet to tip as it's nose is entering the rifling.
Recently a customer who I had sold a .460 diameter bullet to, for his 45-70, could not make it shoot, and I could not give a solution. Till on his own, he tried sizing it to groove diameter and presto! Tight groups.
At least 15 years ago, a customer sent his throat slug from a 30-30 contender, which he couldn't get acceptable groups from with any of several molds I had made him for other 30 caliber guns. I told him to size close as possible to groove diameter and heat treat his bullets to max hardness. Bingo, groups were tight, with occasional flyers going out of the group, slightly, but not bad, considering the gun. He went to a turkey shoot the following weekend and brought home the turkey!
I have been and will continue to recommend chamber filling bullets for 45-70, 30-30, the revolver ctg carbines and similar chamberings, because most chambers are quite concentric with the bore. However, if you make a slug, at least check it visually, or if you are having trouble getting groups, and don't know what else to try, make a throat slug and give it the visual check. Doesn't matter the cartridge chambering, if the barrel isn't centered with the bullet as it comes through the throat, it's going to hurt performance.
