NEW YORK (MarketWatch) -- Shares of bond insurer Ambac Financial Group Inc. /quotes/comstock/13*!abk/quotes/nls/abk (ABK 0.53, +0.01, +1.53%) were up as much as 29% Friday on heavier-than-usual trading. At the latest, Ambac's stock was up about 22%, at $1.16 each, with nearly 39 million shares traded. The 30-day average volume is about 31 million, according to FactSet Research.








