a little off topic but what the hell it is information I personally learned the hard way.
back from the 90's into the early 2000's, military ranges (at least the ones I was on) only offered the over ear defenders, not the nice new miked ones.
any how,
according to my audiologist, they do not seal properly for people who wear glasses (me), and can move & loose their seal with certain people mixed with certain rifles, do to cheek weld.
I only use foam these days
This is a really, really good point. I recently took my Sub2K to the range with my son, and there were also a couple of range members that wanted to shoot it. Based on the aggressive cheek weld and the stock position, etc. of this thing, it the butt kept bumping onto our over the ears muffs - not just me, but my son and two members of the range as well. Previously I used earplugs, and was surprised by how often I had to adjust the muffs before taking the shot. I think you are probably right in that even with what we would think is satisfactory adjustment, that the muff may not be 100% sealed due to the gun hitting the muff exterior no matter what (we just reduced the degree of the contact with our adjustments). My son and I both wear glasses (not just shooting glasses, but prescription glasses too) and both people who shot the gun also wore prescription glasses.