The differentiation was created for many reasons, especially to avoid the confusion that was previously created by referring to all MSRs as "ARs" or "AR-15"s. The prior terminology was problematic for many reasons, including:
- AR is an abbreviation for ArmaLite Rifle, which is a specific manufacturer of MSRs (now owned by Colt). This misrepresents the devices which are made by numerous non-ArmaLite manufacturers including Bushmaster, Smith & Wesson, Stag Arms, and many more.
- Calling some MSRs "AR"s led to a common misconception that it stood for "Assault Rifle" which, in reality, describes weapons that are fully automatic or burst-capable - unlike the modern semi-automatic MSRs.
Many features that are commonly found in factory-produced MSRs are:
- Semi-automatic functionality
- Rail-systems for adding additional attachments
- Adjustable stocks
- A pistol grip
- Detachable magazines - most commonly 30 round capacity