Cutaway MAC-10, showing off the Sionics suppressor with a compartment filled with shoelace eyelets.

https://lemmy.world/post/45137111

Why shoelace eyelets? Easily available and they just worked?
The eyelets randomize the motion of the expanding gases which reduces noise. They probably absorb some thermal energy and act a bit like a heatsink.
Wouldn’t they make the gun rattle a lot when walking around, though?
Not enough. They’re packed in tight enough that they don’t rattle around much.