I don't know, the gunner's mate said it hit a rock, and other Swiftboaters said it was a common wound (that they all got stung once or twice).
Notice in the book it doesn't give an exact range, so you can't really tell. Shrapnel sure can fly.
They explode on impact
http://www.vietnam-war.info/weapons/m79_grenade_launcher.php
Rotation of the grenade armed it after about 15 meters and the shrapnel tarveled at about 1500 meters/s.
I was wondering what the air speed velocity of unladen rice is.
"..My recollection is that the spin-setback arming sequence distance for all M-79 rounds was 33 meters..."
I believe someone told me a long time back. that there are two types of projectiles. One
for use out in the field with a long arm time. And one for short close in door to door
fighting with a very short arm time. But I don’t remember for sure.