Ross and Bill,
The approach I’ve followed is to measure the neck wall thickness and measure the OD of the sized cast bullet. Add the two neck thicknesses to the bullet diameter and we get the calculated OD of the loaded round. (Similar to what Bill noted.) Then to double check that, I assemble a dummy round and mic the OD of that round. The two results should be, if not the same, very close to the same.
That is compared to the neck ID. This tells me how much material needs to be removed when outside neck turning. Hopefully, some material needs to be trimmed from the necks. If I’m lucky, the loaded round OD is just small enough to provide that minimum clearance of 0.0005” per side. If I’m unlucky, the loaded round OD will be too small to provide the tight clearance that is desired.
Over the years I have found that Lapua brass is the most consistent brand in the neck wall thickness area.
It’s just me but thinning of the necks to a uniform thickness, would be more beneficial than inside neck reaming. However, as Mr. Alexander would say…” show me the detailed tests and scientific backup”.
When we are sneaking up on a tight necked chamber, we are trying to minimize case neck expansion as a result of firing and maintaining a very close dimensional relationship between the OD of the loaded round and the inside surface of the chamber’s neck area. Having the neck wall thickness of every case being the same, helps us maintain consistency in this regard. And, as Ken noted, concentricity of the bullet to the bore, just before firing, is aided by the uniform neck thickness and the tight loaded round fit in the chamber.
For a factory production rifle, that does not employ a “tight chamber” we are dabbling in the pool of diminishing returns. I have a Savage Model 12 chambered in .308 Winch. I bought (200) Lapua cases and just started load testing. Turning the necks never crossed my mind. While I didn’t attempt to measure the ID of the neck, I did learn that it was quite a bit larger than the OD of a loaded round. It might have made me feel better to turn the necks but I save that exercise for custom, non-production, often times wildcat round match rifles.
Years ago I had an XP-100 chambered in 30 BR, a close cousin to Ross’s 308x1.5, using cast bullets in handgun silhouette matches. (I can recall someone getting some forming dies to convert .308 cases to 7BR, with a bunch of us sharing the dies. I can’t recall anyone doing any neck turning. This was when the 7BR caught fire with silhouette shooters and factory 7BR cases were not yet available.) At the time, 1990, someone asked me if I turned the 30 BR necks and I said….whats that?
One after thought…the machinations of the jacketed bullet benchresters go through are of no interest to me. Their game is a lot more “monkey see, monkey do” than the CBA shooters engage in, thankfully. Take a look at the jb’s equipment lists….lots of similar components, guns and scopes.
For our CBA match game the match is won @ the bench. These case gyrations are helpful but are not the key focus. We need to go look in the mirror…