LineFormat Cap Type COM API
Lines can have different cap types ("Square", "Round", "Flat") in PowerPoint, but that setting is only available interactively (in the user interface). The LineFormat COM object does not offer this option, and I couldn't find any other COM API for that purpose.
What makes things worse: Some dash styles like Office::msoLineSquareDot and Office::msoLineRoundDot implicitly change the cap type. When the dash style is changed again, the cap type sticks but for the programmer there is now way to tell its current state.
Volker Schöch commented
(Internal reference: RT#11451)