Originally Posted by
Puffin
Further to my earlier post, I've since cross-checked several dimensions on the Manson and JGS regular SAAMI reamer drawings.
The neck diameter runs from 0.323" to 322". Necked up but unturned 6.5-.284 Lapua brass measured 0.3120 - 0.3126" loaded, so that is already 10 thou expansion on firing. If the brass is then turned and tidied up for a donut then 0.310 is about right. That is quite a lot of to-and-fro for each firing/resizing cycle. Online reports indicate that the Norma .284Win brass has thicker neck walls than either the Winchester or necked-up Lapua brass at 0.316" give or take. This might be reason enough to use Norma brass in a standard chamber, but also a potential safety issue if the chamber is cut with a non-standard neck.
The SAAMI profile does NOT include parallel sided freebore. It tapers down from 0.290" ahead of the neck at 47' 33" (0.7925°) in one continuous freebore/leade. At that angle is narrows to the nominal bullet diameter 0.284" at 0.217" ahead of the commencement of this profile, so this figure can be substituted roughly for freebore. It looks like many here are getting good accuracy from their .284s, but based on this I don't imagine many of those bullets that aren't 180gr are out near the lands.