Obturation , PRB and rifling

[Dean2]

The bump-up of a bullet may PRODUCE the obturation of the bore, but this deformation of the bullet is not the obturation itself, but is instead the mechanism producing it.

This I absolutely agree with you on, but the point remains that either the ball or the patch and ball, or bullet is what obturates the bore, or not: if it doesn't block off the gasses properly.

 

[Joel/Calgary]

Dean, I reread the information you quoted, and they are actually using "obturation" correctly, but in a context where upsetting of the bullet is the only mechanism in play in sealing the bore (although I do suspect that engraving on entering the rifling may actually be involved in some of the cases).

obturation is the result

and they refer to both obturation of the bore by expansion of the bullet and of the chamber by the expansion of the case neck. They do separate the mechanism from the result, but with phasing that does not make that separation as clearly as might be desired.

Regards, neighbour,
Joel