Chamber neck size vs. brass neck dia.

Ok, this kinda ties into my post about a sizer bushing for a 260 AI, but i really can't figure out the #'s. If a chambers neck dia is .298, as in the 6.5x284 Win. and the Norma 6.5x284 is .300...(per another post) why is the new brass neck dia , both lapua 6.5x284 and Rem 260 measuring .291 loaded? Everything i read , and all of the help i have gotten says to neck size .002 under a loaded rnd.s od neck dia.That figures out to sizing the neck around .009-.011 each time......is this the norm? Some have said no, but alot of what i read shows these #s (.298 neck chamber) as the spec....been in this game awhile, and i'm STILL learning new facts every day!!!!! Thanks for any help....rsbhunter

You need to measure your brass.
For best results you need to use a ball or tubing micrometer. It don't have to be expensive. I got mine off eBay for $20 some bucks.
Read this thread.
http://www.6mmbr.com/casenecktools.html

For example if you have brass that when measured the neck diameter is .010 you multiply this by 2 so .010x2 = .020 . Take that .020 and add your bullet diameter to that. (6.5 MM = .264) .020 + .264= .284 this is what your loaded round should be. Then for your bushing size take the .284 - .002 and get .282 this is your bushing size.

Now if your loaded round is measuring at .291 then your brass neck thickness should measure at .0135 or 13 and 1/2 thousands. (.291 - .263 = .027 divided by 2 = .0135) Bushing needed would be a .289

Now for optimal fitting if the chamber measures at .298 you will need .002 to .003 minimum Clarence between the round and the chamber for expansion. (Old wives tale is benchrest shooters like .001 Clarence, fact is with chambers that tight it causes unexplained fliers and other problems.) So with that said, You want your loaded rounds to measure at .296 to .295. So to get that loaded round you need the necks thickness of your brass to measure .016 to .0155 thick. (.296 - .264 = .032 divided by 2 = .016) I would say this is a no neck turn chamber.

The fact is if you brass is undersized it will not affect accuracy, but it will work the brass more, and cause it to live a shorter life. I doubt that your are sizing the brass with .009-.011 neck tension, I would say your brass has a smaller neck thickness that your are accounting for.

It could be the brass , as it is Rem factory new...the Lapua 6.5x284 brass is thicker...might think about either buying Lapua 260 brass, or neckong down 7/08 or 308 brass, as that should also increase neck thickness....Thanks for the help, rsbhunter

if you have a loaded round at.291 and your neck is .298 then your good to go if you want you can use a .289 bushing .002 under loaded