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

https://lemmy.world/post/45137111

That second pic looks straight from Soldier of Fortune magazine.
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.