Rock River Arms is owned and operated by brothers Mark and Chuck Larson. Between 1981 and 1991, the Larsons worked at
Springfield Armory, Inc., where Mark was the head armorer. From Springfield Armory, the Larsons went into partnership with Les Baer to form
Les Baer Custom and build custom 1911 pistols. Parting ways with Les Baer in 1993, the Larsons began to build AR-15 type rifles for Eagle Arms in
Coal Valley, Illinois. At around the same time, they began their own venture, Tolerance Plus (the name was later changed to Rock River Arms), producing custom 1911 pistols. The Larson brothers severed their connection with Eagle Arms in 1997 when that firm moved to
Geneseo, Illinois. At the same time, Rock River Arms began to build AR-15 style rifles. Rock River Arms also produces
M1911 pattern pistols,
bolt-action rifles, and
AR-15 style pistols in assorted calibers. ==Law enforcement use==