What are the differences in the WSM cartridges/calibers compared to the older and current calibers?

In a joint engineering/marketing plan, Winchester/Olin (Ammuniton), U.S. Repeating Arms Company (Winchester rifles and shotguns licensee), and the Browning Company developed, based on the patent granted to Rick Jamison, five totally new short-action cartridges. The first three, introduced in 2002, are the 270 WSM (Winchester Short Magnum), the 7mm WSM and the 300 WSM. These cartridges are shorter and fatter than their counterparts. They perform with greater speed and better ballistics.

WARNING These short-action calibers will not fit in magazines or chambers of their counterparts. Also, ammunition in the 270 Win., 7mm Rem. Mag. and the 300 Win. WILL NOT function in these rifles.

MAKE CERTAIN that the ammunition you purchase is correct for the caliber of rifle you have. If you purchase one of the WSM rifles, you must select the proper WSM cartridges to fit it.