If the cap came into contact with the bore, something has gone horribly wrong!

If designing this thing from scratch, one would ensure that the base of the cap is stepped-in from the perimeter of the slug body, by sufficient amount to miss the rifling in the choke. Besides, a shoulder is needed to roll-crimp onto anyway.

The guys in Australia have been shooting plastic wadding in nitro paradox guns for years. Perhaps there is a plastic fouling problem in the chokes, but not to the point of anyone mentioning it. Nothing in Wrightie's book either IIRC.

Not that it's a crime, but Seyfried can take the 'traditional' thing to the extreme sometimes!