how about Shrapnel Ballista of Steel?

I found it have more base damage and base impale chance, the only one bow skill that could fully use steel shards.
But I don't know well about this skill, need some answer.

1. Is call of steel enough to let 7 Shrapnel Ballista always have shards to consume?
if not, have exrta way to replenish steel shards?
and would using call of steel too continually reduce my DPS? because it would consume all impale stacks to damage other enemy around target, not target itself.

2. Shrapnel Ballista seems could shotgun, but it have only 3 projectles and it looks like only a line not fan-like.
should we to get more projectles or put it closer to make it easier to deal shotgun?

3. is Shrapnel Ballista of Steel really better than Shrapnel Ballista or Siege Ballista?