Remington Ultra Magnum, or RUM, refers to a "family" of cartridges developed between 1999 and 2002 by Remington Arms.[1] All of the RUM cartridges are based on the .404 Jeffery non-belted magnum cartridge. There is a long-case line, as well as a shortened version designed to fit a short rifle action (such as a .308 Winchester). The long case is very large and provides performance that exceeds existing commercial magnums, such as the .300 Winchester Magnum and .300 Weatherby Magnum.[2]
The short design is the same idea as used in the Winchester Short Magnum cartridges. The shorter cartridges are known as Remington Short Action Ultra Magnum, RSAUM, RSUM, or SAUM.
The RUM family of cartridges are Delta L problem cartridges, meaning they can present unexpected chambering and/or feeding problems. The Delta L problem article explains this problem in more detail.