Rock River Arms is owned and operated by brothers Mark and Chuck Larson. Between 1981 and 1991
the Larson’s worked at Springfield Armory, Inc, where Mark was the head armorer. From Springfield Armory, the Larson’s
went into partnership with Les Baer to form Les Baer Custom and build custom 1911 pistols. Parting ways with Les Baer in 1993, the Larson’s 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.
Around the same time, Rock River Arms began to build AR-15 style rifles. For a time, Rock River Arms also produced M1911 pattern pistols in addition to the AR-15 rifles, however eventually they dropped that line and now focus their production
exclusively on AR-15 and M4 pattern rifles.