Update #1

Didn't work.

I used Federal small pistol primers, .38 Spc. brass and Radians foam earplugs.  I think that the earplugs are too big for use in the .357 brass and bore.  I loaded twenty rounds, two earplugs exited the barrel.  Two failed to exit the brass completely, all the earplugs were too damaged to be reused.  All the primers bulged and showed signs of over pressure, as primers were only propellant no damage came to the firearms.

Next experiment will be with .44 brass, but those guns are all in storage so it will have to wait.  Also it looks like I have lots more large pistol primers than small pistol primers.  If I can find my CCI 550s, I still have reloading stuff in at least three places, and I can find some smaller earplugs I might try again with .38 spc brass.

When my camera card finishes down loading to the computer I will post some pics up.

Earplugs by themselves caused over pressure?

Forgive my ignorance, but why would anyone wish to shoot ear plugs? For reenactment purposes? (to get the boom, but hopefully none of the more injurious consequences).

Usually in quick draw competitions, for safety. You still wouldn't intentionally shoot them at anyone.

Earplugs by themselves caused over pressure?

The primers showed signs of over pressure.  There was no other propellant but the primer itself.  Here was my process;  I primed the brass, squeezed the ear plugs put them in the case, and them went out and shot them at a no parking sign on the shed wall.   

A couple of the plugs expanded tightly enough in the brass that the primer ignition blew the plug in half, but not out of the brass.  Most of the earplugs had holes blown partially threw them.

Brass was found in a box of reloading stuff de-primed, but not resized, and very dirty.  Also guns' bores were not shiny clean.  I think when I try this again I will use shiny clean nickle plated cases.  I will use a caliber and a earplug sized so that the plug fits snugly in the case, but does not need to be compressed to be inserted.  I will have the bores of my test guns shining like mirrors.  I might have to get another sample pack to see what plugs work best.

Thanks.