but the bullets in this case were sent to kill, i.e. head shot... if they were sent to capture Hiruzen, sure. You could say Oro was toying with Hiruzen all day long, but the goal here was to kill Hiruzen using his former mentors.
Imagine I wanted to kill you but did't put in a full effort.
What if I had you tied up, and I knew you would die. But I wanted you to suffer. I shot wildly
If his goal was to kill, maybe he wanted to kill him using minimal effort.
I could challenge myself to kill you bu only hipfiring. Or choose to only aim at your non-vital spots until you died a painful death.
Anyway, knowing Oro's character, it's quite possible he was toying from the start. No manga page saying he wasn't.
For those who don't get my analogy - or think I'm going off topic - I'm saying
Just cause a weapon is perfected, doesn't mean the user is perfect at using it. Nor does he choose to perfectly use it.