Do you mean like Ground Branch or Tarkov?
Where you can manually place the position of gadget anywhere along the Picatinny Rail (Ground Branch), or any compatible slot (Tarkov)?

Idk how Ground Branch does it, but last I checked a Tarkov weapon, their system was fairly similar to Payday 2's in having rigged every single attachment point available to the gun.

So if you'd want vanilla Payday 2 weapons to mount a gadget position to each handguard, someone would need to be insane enough to rig every single applicable attachment point you'd want for customization. Optics have an additional layer of annoyance when having to ensure every optic is zeroed correctly to whatever other optic attachment points you define.