You're absolutely right about the 9mm. It slipped my mind and now I feel a little sheepish since Vietnam is my area of expertise!
I have seen numerous pictures of guys in the MACV-SOG teams using this weapon. When stealth is paramount, this weapon definitely fits the bill.
These guys regularly were operating out of the VC/ NVA back yard in Laos, Cambodia, and even North Vietnam. If they were engaged, it was able to put out a wall of bullets to give the enemy pause so they could break contact and quickly get out of Dodge!
The Hush puppy was invented specifically for these types of missions and they were put to good use in the jungles and paddies of SE Asia. I would have to say, if it worked for them, then by all means if would work for me!