Re: 300 win mag is splitting cases.
You've got yourself a headspace problem there, even if your headspace gages out just fine; this is one of the quirks of belted cases, and why the newer family of magnums (like the RUMs, the SAUMs, WSMs etc), have done away with them. They were needed when the original magnums were developed due to manufacturing capabilities of the day, and their inability to control headspace of those long, tapered cases like the 300 and 375 H&H mags. They actually weaken the case, from an engineering standpoint, and complicate the reloading end of things today. For a case of modern design, like the 300 Win, they're nothing more than a decoration. Your cases are stretching within your chamber, and that's what's giving you the separations and/or cracking. With it's doing this on the second firing, it'd be worth having the gun checked, but it may be due to the action springing slightly, since the T/C isn't going to have as strong of a lockup as a bolt gun. Beyond that first firing, take the advice BigSky offered about neck sizing, or at least, adjusting the dies to avoid setting the shoulder back anymore than need be for the rounds to chamber. Normally, if you treat the belted rounds like normal rimless designs, they will last a lot longer than if you continue to headspace them off the belt. Hope this helps, but let us know how it works out for you.