The laws concerning machine guns and short barrel rifles are defined in the National Firearms Act. There are some stipulations concerning sear usage with short barrel guns. This information assumes SBRs are allowed in your state.

Registered sears can be used with any guns in which they were designed for. They are often referred to as RDIAS, registered drop-in auto sears. Common sears are often found for M16/AR-15 style rifles or HK roller locking guns like the MP5/HK91. The sears will cost nearly as much (sometimes more) than native machine guns. However, there is a difference in classification between machine guns and auto sears.

A machine gun is considered automatic and eliminates the requirement of the short barrel rifle tax stamp. If you own a machine gun on a form 4, you are not required to have a form 1 as well if your gun has a barrel shorter than 16 inches.

In contrast, if you own a sear, you may add a stock to your gun therefore making it a short barrel but only when the sear is installed. However, once you remove the sear you must remove the stock and return the gun to a 16" rifle or pistol configuration.

Sometime a machine gun and sear are sold together. This is referred to as "married." If you have a machine gun with a married sear you may swap the sear into a different gun with some requirements. By doing so, you are required to submit a form 1 for the original gun if the barrel is shorter than 16 inches. This would allow you to remove the sear for use on another gun, but still posses the original gun as a short barrel rifle. Moving a sear from a married gun without a form 1 is considered illegal.