Re: 7mm WSM vs. 6.5x284
the lapua brass i have bought has had more consistent neck wall thickness and better factory neck annealing than winchester and therefore more consistent neck tension than winchester brass, that will/does improve accuracy [you can-and i do-turn the winchester necks and after enough work it shoots well enough] , you may want to read the excerpt below from the 6mmbr web sight. i consistently get 1/3 - 1/2 moa groups with my LSR 7wsm out to 500-600 yards and it shoots very accuratly out to 1k, i routinely do better out to 600 yards with my 6.5 x 47 running lapua brass [both using berger vld's];
the nice thing about opinions is that everyone gets to have one...
Neck Variation in Brass -- Why You Need to Measure
Extreme accuracy demands consistent case necks. If your case neck walls are thicker on one side than another, your bullets can be positioned off-center in the chamber, or tilted slightly off the bore axis. That means your bullets can actually start off crooked as they enter the barrel. (And "jamming" bullets into the rifling won't necessarily correct the problem.) If some of your case necks are significantly thicker than others, it will be difficult to maintain consistent neck tension from one round to another. That can kill accuracy in a number of ways. Likewise, widely varying neck tension can cause nasty swings in cartridge pressures and velocities.
Take any two cartridge cases out of the box, and you will see variations in neck wall thickness and uniformity, even with superior brass such as Lapua. With low-quality brass, the variations may be extreme. We've seen Remington brass vary up to .004" in wall thickness within the same box.
The illustrations at right show the two basic problems with neck walls. First, the neck walls on some cases, even if they are uniform all the way around, will be thicker than on other cases. This hurts accuracy because it is virtually impossible to maintain consistent neck tension with brass of varying neck wall thickness. Yes, you can try to adjust tension using different bushings, but it is much better to start with brass that has the same wall thickness.
Even if a batch of brass has the same average wall thickness, give or take a thousandth, you can still have problems if the neck walls are not uniform. This is illustrated in the second diagram. Neckwalls can be thick on one side and thin on another. The only way to "cure" this problem is to turn the necks.
Of course, in a given lot of brass you will probably see a combination of these variations--both case to case differences in average wall thickness and lack of uniformity on individual cases. That is why it is so important to measure your case necks, even if you shoot no-turn necks. With a good case neck wall measuring system you can cull the worst offenders, and then segregate your brass according to neck thickness.