Issuance of securities backed by adjustable-rate mortgages gained considerable steam during the first half of 2004, accounting for more than a third of the period’s total MBS volume, a new Inside MBS & ABS analysis and ranking reveals. Overall, a total of $318.96 billion of ARMs were securitized during the first six months of the year, a 55 percent rise from the same period in 2003. That strong performance was powered by a surge in…