You want a weapon with a reach of 21" (r1)?
That's about as good as a knife at those ranges.
Forgive my curtness, I have to scurry off to the office, but why entertain what seems like a gimmick?
Stun batons may seem cool to some, but unarmed assailants can do massive damage at 21", heck 21' and you could be in deep trouble. (Tuller Drill (r2a, r2b))
That, and will you really carry a device that's a foot long when collapsed?
(note:
r1) The dimensions I am referencing are based on the OP's mention of the "Streetwise Expandable Stun Baton 700K" whose stats I read from
http://sepharus-store.stores.yahoo.net/ezsdsweb700.html
r2a) Tuller Drill - David Kenik "Armed response" (ASD Publishing 2004) 29-30
r2b)also referenced in "In The Gravest Extreme" (Ayoob 1980) by Massad Ayoob, however I couldn't find the specific citation, (not enough time) though the idea is mentioned on pages 16-17 though not referred to by name as the "Tuller Drill".