Actually, no.and all the short mags
Winchester Short Magnum
An article of importance..All of the WSM cartridges are based on the .404 Jeffery non-belted magnum cartridge which is shortened to fit a short rifle action (such as a .308 Winchester).
Pros & Cons of the .300 Magnums
The belted case was a British idea that originated with the .400/375 Belted Nitro Express in 1905, and it was later used in the development of the .375 H&H and .300 H&H Magnums. Since the .400/375 and .375 H&H cases have virtually no shoulder and the shoulder of the .300 H&H is long and mildly tapered, the British figured all needed a belt for positive headspacing, and they were right. Magnum-size cartridges, such as those developed during the early part of the 20th century by Charles Newton, were exceptions, but other magnums that came along after the 1920s had belts on their cases. While the belt was unnecessary on most of those cartridges, it spelled high performance to many hunters, and for that reason it sold lots of rifles and ammunition through the years. Then came a new breed of magnum cartridges without a belt, and suddenly that little band of brass just forward of the extraction groove of a case had more critics than Michael Moore at a soap-makers convention.