What is the first step in assembling the trigger housing assembly of the M240?

The first step in assembling the trigger housing assembly of the M240 involves inserting the holding notch into its designated recesses. This is crucial because the holding notch provides stability and alignment for the entire assembly. Properly seating the holding notch ensures that the trigger housing is positioned correctly within the assembly, allowing subsequent components to be installed correctly and securely.

Other steps such as inserting the spring pin or aligning holes come later in the assembly process and depend on having the holding notch properly fitted. If the holding notch is not correctly placed into its recesses, the assembly can become misaligned, resulting in difficulty during installation or potential malfunction of the firearm. Therefore, starting with the correct positioning of the holding notch is essential for a successful assembly.