I am following this with interest - a learning for me, I think? Had read Veral Smith's writings about wanting cast bullets to snuggly fit to the "ball seat" or "free bore" area- between the end of the cut in the chamber for the cartridge neck and the beginning of the riflings - the leade. Then, "normal" casting wisdom seems to call for a cast bullet to be sized about .001" larger than the barrel's groove size.
The serious bullet casters ignore the hoary old wisdom of ".001" over groove size". They're in general agreement with Veral (or he's in agreement with them, as some pre-date Veral's pretty good book). The closest you can get your bullet to fitting the ball seat/leade, means it's that much less bullet body gets moved around during obduration. And it's much harder for gas to escape around the base of the bullet prior to the bullet fully obdurating and sealing the bore. That leads to bad things if you care about accuracy.
To underline that, more than a few competitors in cast bullet benchrest start out with a mould that drops bullets that closely match the dimensions of the ball seat/lead when seated. Then they take those bullets and swage them to their final shape in a swaging die that replicates the ball seat/leade cut in their rifle. It's got nothing to do with whatever groove diameter is or isn't. The swaging die, properly made, also does its best to ensure the CG is right on the axis of the bullet, not a little off to one side or other. With more than a few of them, their rounds fit so tightly into the ball seat/lead that if they have to unload the rifle, the case comes out but their bullet remains inside, seated into the rifling.
Benchrest is a bit specialized, obviously, but the point is they believe they get best performance the closest the bullet fits into the ball seat/leade. That also works for hunting, other kinds of matches, etc. Guys just don't go to the same extremes concerning bullet fit.
Had read that slightly larger makes not much matter - will be simply swaged down as it passes through the throat, but may be a revolver cylinder thing, more so than for a rifle.
It MIGHT not make a difference (then things worked out for that guy), but it certainly CAN make a difference.
Then you can get the opposite: My grandfather's 1895 Winchester chambered in 30 U.S. has bore and groove diameters that belong on a .303 British. Cast/size bullets to .001" over groove diameter, seat them - and you'll discover your reloads will not chamber in the rifle... it still has a 30 U.S. chamber, so with a .315" bullet for that groove fit seated in those cases, now the measurements at the neck of the case with the bullet seated are too big to get in the neck portion of the chamber. The solution is to choose a bullet that gives a good fit in the ball seat/leade, and then size down the portion of the bullet that seats in the case, small enough that when seated the neck of the round will now have just enough clearance at the neck to chamber.
So it's not unlike a heeled bullet like you have with .22 rimfire: the portion of the bullet that is inside the case is of a slightly smaller diameter than the bore riding section of the bullet outside of the case neck. A heeled bullet design for loaded ammunition was fairly common back in the days when it was all cast bullets.
As a general rule, I want my cast bullets to fit as tightly as possible in the neck/ball seate/lead. If it just barely has enough clearance to seat in that area, then obviously you don't have much obduration needed to seal the bore, and less deformation takes place during that deformation. I'll take a bullet that only has to obdurate .001" to seal the bore over one that has to obdurate .005" (or whatever) to seal the bore.
It isn't any harder to do and it doesn't make it longer to assemble your reloads. It just takes more time the first time, figuring out the fit and then getting your dies set up so you do the same thing every time you reload after that.
"At best (or worst?) might require thinning cartridge necks to be able to chamber, but that sort of presupposes that all the bullets are the same diameter??"
If your bullets vary in dimensions, bullet to bullet, then you're probably going to have a problem getting decent results, no matter what kind of farting and tapdancing you do in other areas of the bullet/case prep and reloading sequence. Uniformity in bullets matters in cast bullets just as it does with jacketed bullets - at least as far as accuracy goes.
One option, the easiest, is to only size the portion of the bullet that fits within the neck by base sizing, sizing the base just enough so that after the bullet is seated, the case neck with the bullet seated has just enough clearance at the neck to chamber. That leaves everything forward of that untouched, and presumably giving you the fit you want in ball seat/leade in front of the chamber neck.
NOE's neck expander plugs and bullet sizing setup are both the cat's pyjamas when you decide you want some precision in both neck expanding and bullet sizing. And they are relatively inexpensive, as well as available in .001" size increments.
https://noebulletmolds.com/site/product-category/expanders/
https://noebulletmolds.com/site/product-category/sizing-tools/sizing_kits/
Turning case necks is another option: if you use one of Lee's collet sizer dies that you've specified the right size mandrel for, then the metal you remove from the case neck can be done by outside neck turning, and because your mandrel gives you a uniform I.D., then all the excess metal to be removed will be on the outside of the case neck - you should end up with uniform neck thickness all around the neck, which might also improve accuracy in a good rifle.
I do not know if you have some, but I found a Cerrosafe casting of the chamber's neck and the first bit of the rifling was a very handy way to get some accurate measurements of what you have going on within your rifle. It may all be fussy work for nothing - the projectile is going to emerge from the muzzle in the shape of the bore, and closely sized to it - regardless of what it started out as - so one of those things to have to actually shoot it and see?
Wondering about cast bullet dimensions when you don't have the measurements of your chamber/ball seat/leade is kind of pointless unless you don't mind simply trying trial and error to see what happens.
As an example, if I hadn't bothered doing a cast of that 1895's chamber (even if I didn't cast bullets), how would I have known it had a barrel with .303 British bore and groove dimensions, instead of proper .30 caliber dimensions? Things had worn a bit over the last 123 years (with some of that time involving corrosive ammunition)... Once I did a casting, it didn't take long to figure out why it grouped like a shotgun with buckshot with 30 caliber jacketed bullets. Started loading jacketed bullets intended for .303 British and group sizes were immediately halved. When I went further and fitted properly sized cast bullets for hunting, group size got another huge improvement. I don't bother loading jacketed bullets for that rifle anymore; I get better accuracy and just as good performance on game with cast bullets.
https://i10.servimg.com/u/f10/14/11/62/01/5-8610.jpg
A pound cast is better than Cerrosafe, simply because the dimensions never change once it is done. Cerrosafe's dimensions do change slightly over time, but if you measure the casting up at the proper time after you've removed it from the chamber, I guess that doesn't matter much. You can always do the casting again in the future.
