For your interest, here's the attenuation of my latest set. Red line is right ear without plugs, blue is left ear without plugs, grey is with plugs in but turned off. Vertical axis is dB, horizontal axis is frequency (Hz). My audiologist and I did this for interest I picked up my new set. It's just done with standard tones rather than simulated gunshots or anything, and it's a test of the fit effectiveness rather than the electronics. But as you can see, they block pretty darn well (and they're made of the hard plastic which you mention).
Attachment 273280
And a picture of the plugs themselves.
Attachment 273284