I've been away for a couple of days, but I wouldn't be concerned with three thousandths, that's actually pretty nice. I agree you should KNOW your neck diameter on the reamer, if its a 255 and you have an actual 256-257, that's still pretty within good tolerance.
Couple more things:
Loadeds are 252? With a 251 unloaded but sized? Fired 253? From that --
If the neck is 255 nominal, and fired brass 253, that's about right taking "spring back" into consideration, brass bloats upon full pressure and then springs back smaller once the gas is out the barrel. That's why we can extract cases normally, at NORMAL pressures, which do not exceed the elastic limit of the brass (and barrel steel to a certain degree). Super hot loads not only bloat the brass, but also the barrel steel, and when the pressure returns to zero, the brass molded itself to the bigger steel, and to your extractor/ejector cut. Bad juju, that.
So I like your numbers so far.
One number you'll need to memorize now is 255, and loaded diameter of neck at 252, MAYBE 253 if you really want to get precise. But 252 should be our target for loaded necks, anything fatter needs turning to make SURE the brass comes away from the bullet upon firing and bullet "release" into the rifling. So, whenever you get new brass, just stuff in a bullet and measure your finished outside diameter. If it is more than 252, then it's smart to outside neck turn, and to turn all the way down to the neck-shoulder junction to make sure the INSIDE diameter is the same all the way from top to bottom and the bullet isn't being collared by "The Dreaded Doughnut." Basically, a ring of brass that acts like a choke collar on the bullet when chambered. A bad thing.
As things are, as long as your fired rounds finish under or at 252, you're in good shape.
Inside versus outside? Inside doesn't follow the brass variations. Outside first, and THEN maybe with the right inside reamer for "dressing" and to zap any possible doughnut evil. Doughnuts are evil. If your necks are icky and lumpy, outside reaming will show you that right away. So the steps are:
Initial size with expander ball, to make the neck round on the inside (it follows the last thing it touches, the ball).
Outside ream, but only enough to make things round. And if you do this, you have to do all the cases in the batch exactly the same
There should NOT be a doughnut at this point unless you are necking something down or changing the shoulder.
You should have a neck that is round and consistent both inside and outside.
Now our issue is how tight do we want the neck to be on the bullet. You can control that with your bushings, very precisely. I have 247 through 253 neck bushings for my 22s. I use the 248 and 249 the most, which translates out from fired to sized of about four thousandths, ie, fired 252 gets a 248 and ends up 251 or 252 loaded. So that's "four":thou of "grab." How much is right depends, but three or four is normal for my rifles.
With your 251 sizer, that's two thou of grab, which might be less depending on how springy your cases are. I would get and try both 249 and 250 as a start. The concept you need to think about is, if there's a half-thou of variance, a half of 1 is 50 percent. A half against 2 is 25 percent, a half against 3 is 16 percent -- the idea is consistent pull without mashing your brass down and working it too hard. Too much is when you can see where the bullet stops in the neck, it measures fat where the bullet is, and skinny behind the bullet and in front of the shoulder.
"Pull" is important at the start of the ignition cycle. The powder starts to burn, and the rate of increase of the pressure should be a smooth curve. If there's not enough pull, the bullet moves (Mic McPherson wrote about this many times) before it should, then bumps up against the rifling, and there's a little blip there that affects final velocity. How much depends on the powder and how the bullet is seated in relation to the throat. But it can be a lot. So, consistently starting the bullet at the same point in the ignition cycle is a goal of all this.
Anyway, I think you're going to be real happy with your new chamber.
