When using a heavy bolter on full auto (ROF -/-/6) , with metal storm ammunition (Adds blast 2), is the maximum hits to a horde 19 (6 regular hits for normally hitting the horde, 6*2 blast hits, +1 explosive damage type)?

Or does the explosive damage type of the heavy bolter apply for each individual hit, rather than the entire full auto action.

That would put the entire number of hits to a horde up to 24 (6 regular, 12 blast, 6 from explosive damage type).

Or am I reading the rules wrong and there's a different variation?

One additional Hit per attack after all other hits have been applied.

1 Full Auto Attack = 1 extra hit.

Unless Blast changed from Deathwatch, it would actually be 13. 6 hits from the Heavy Bolter, each of which is turned into 2 by the Blast effect, and then +1 for being explosive.