the red primers holders are just an touch larger the plunger. I can see where they are hitting. with out pulling out my caliper, i am going to say any where from .05" to .1" larger.

I have the breech plug packaging in my hands. it is for the gun. it may be the plunger.

oh well, I picked up two spit fire nipples at an local gun shop. see if I can keep from loosing them