I have a question about the neck diameter when I resize my .308 cases. I am using the RCBS regular dies(not the neck bushing type). The Hornady manual show that the neck should be .343, after firing they are at .343 and i put them through the die and they come out at .334?? Am I doing something wrong??

Measure your brass neck thickness take it times two and add the diameter of your bullets. This should come close to the .343 dimension. The case needs grip (neck tension) on the bullets hence they are undersized and when the bullet is seated it expands back out gripping the bullet firmly. This consistant neck tension is necessary for uniform powder burn and preventing set back of the bullet which contribute to accuracy.

Roy,

Welcome to the forum. Rules are to join in and be polite. You'll find lots of folks with lots of information here that they are willing to share, but you are on your own to sort good opinions from bad. As the late Allan Sherman once sang: "My free advice costs nothing and it's worth the price."

The neck of your brass actually stretches out during firing to fill the chamber. It springs back a little, afterward, and so the 0.343" you measure after firing reflects having stretched open to fill neck area of your chamber and then sprung back a little. Stretching out to fill the chamber under pressure is how a case neck lets go of the bullet. The SAAMI specified maximum safe dimension of 0.343" for a loaded round is what is in the Hornady book. That your fired cases hit it exactly is a coincidence.

SAAMI expects the case neck stretched over a 0.308" bullet to have a wall thickness of 0.0175" or less in order to be able to open up successfully in a standard SAAMI minimum dimension chamber. If it were too tight, the bullet would get stuck, possibly hard enough to blow the gun up. Manufacturers like a margin of safety, so most .308 brass is actually manufactured with a neck thickness of around 0.014" to 0.016", so with a .308" bullet stretching it wider, you get about 0.336" to 0.340" O.D. when loaded. The sizing die will put it two to six thousandths smaller to get the grip on the bullet that Hailstone mentioned.

thanks Nick for your help, I have been a lurker for a while and am glad to be able to finally join in. I was concerned as to the effects on accuracy as well as danger, but what you described is exactly as it is with my cases.