Re: Case& barrel life of 300 WSM vs. 7WSM
I'll try to hit the high points in some sort of order that MAY make sense.
First, there really are very few cartridges that are "overbore" any more. With the selection of powders today, read incredibly slow, it just isn't so. We may run out of barrel, but there are usually suitable powers.
As far as barrel life, that will depend entirely on your shooting and cleaning habits. Rapid firing (heating things up), pushing the envelope, and too little, too frequent, or improper cleaning will also shorten barrel life. There is some good evidence that some of the new bore treatments can make a difference in cleaning and barrel life. Do a search... Microlon and Ultra Coat for example. Between your choices, I'd have to give the nod to the longest barrel life to the .30 caliber over the 7mm. A rough guess would be somewhere about 50-100 rounds. IOW, it won't matter enough to be significant, all other factors equal (and they never are).
To address your brass life, in short, there is something wrong. There is no GOOD reason a prudent loader shouldn't get close to 10 firings from a quality brass case. Pushing pressures and overworking cases are the biggest culprits. Use a chronograph, and keep speeds where they should be, to control pressures. Speed equals pressure. Again, that is a generalization, but it applies.
Finally, to address your neck splitting problem. You need to measure... The diameter of: new brass, loaded brass, fired brass, sized brass (without expander), and sized brass with expander. I'm gonna bet your widest spread - between fired brass and sized but not expanded brass - is gonna be pretty large. That is where you are overworking your case necks. Keeping this spread to a minimum will increase brass life. CORRECT annealing also helps, but too little or too much (cooks necks) isn't gonna get it. If you can keep your neck movement within about 4-6 thousandths, and correctly anneal, you should have no more trouble with necks splitting.
Hope this helps................Bug