Been my experience that is true many times but not always. My 1960's Win 30-30 is a perfect example, didn't get it to shoot until I slugged the bore. It has a nice bore from one end to the other evidenced by the bore scope plus no loose or tight spots judging by feel bore slugging or from a snug patch. I was sizing at .310" and most bullets weren't sized with that die, lubed & check seated is all. The catch was the .310" groove diameter and I didn't have a mold that cast that large. I had Erik open up driving bands on a couple of molds and bought a couple of molds that cast large enough. Decent shooter now that bullets actually do fill the bore.
Just goes to show that with most things there just isn't an across the board one size fits all answer to a lot of things.
That's fine, Rick, there ARE multiple ways to do everything.....but maybe I'd better do more explaining of my point of view on this here. Bore slugs get you sorta kinda close and a chamber/throat cast gets you close to exact the first time with no guessing. I realize I didn't mention making a cast as the way one would determine throat entrance diameter, sorry about that, I'm so used to doing that myself and talking about it on the internet that I forgot to mention it specifically here. Chamber/throat casts work every time, bore slugs get you in the ballpark, at best. So I still maintain
and recommend that the chamber/throat cast method of bullet fitment IS an "across the board one size fits all" answer to the question "what size bullet does my rifle need". I will remain pretty much committed to this point until someone can demonstrate to me a better, faster, more direct way to take ANY given rifle and in a few minutes know what bullet mould and sizing die need to be secured and not waste a bunch of time, money, and components trying to guess what will work "best". There are plenty enough variables to contend with to find a rifle's happy place without adding three or four bullet sizes and as many shapes to the equation. One might end up making adjustments later in the tuning process after basing initial bullet selection and sizing on a chamber/throat cast, but that will likely only be a minor point.
I don't know of any smokeless-powder cartridge rifle except possibly the M.C. Escher-esque throat of an NEF 38-55 (loaded chamber neck ID smaller than groove diameter) which cannot have its ideal (or nearly so) bullet band diameter determined by measuring the throat entrance of a simple chamber cast. I don't understand your example of your .30-30, because even with .310" groove, measuring the throat (or chamfer at the end of the chamber if it had none) of a chamber cast would have given you the correct sizing information anyway, plus some other useful stuff like what size the nose needed to be and what to expect for seating depth with the bullet design of your choice. Also, a chamber/throat cast would tell you how large of a bullet your rifle can safely fire without neck pinch. Measuring a barrel slug only tells you the smallest diameter of the barrel's dimensions, which are pretty much useless unless you're trying to ascertain a suspected issue with a barrel such as bulge, choke, reverse taper, etc. I'm not saying DON'T slug the barrel, I'm saying it is NOT the best way to select a bullet mould or sizer.
Going off the throat size as a basis to choose bullet body diameter gives you not only a good idea of what size to make the bullet's bands, but also will give the information needed to determine what sort of bullet nose shape will actually fit, and even more, what shape will likely perform well. Going off of only bore and groove dimensions, obtained from a bore slug, doesn't tell you anything about the throat and leaves the rest up to trial and error. You can't even rely on bore dimension for bullet nose diameter or length because the bore dimension of a slug driven through the barrel doesn't account for throat wear. The ONLY thing you can really tell from a barrel slug with regard to how large to size your bullets is *approximately* what your minimum threshold for leading due to undersized bullets will be.
