It's a machinegun... by definition you only need to pull the trigger once in order to hit the target multiple times.
"... A single attack with an automatic weapon fires 10 bullets. An automatic weapon cannot fire single bullets that target one creature. When taking a full-attack action with an automatic weapon, you can fire as many bursts in a round as you have attacks..."
Even if you can only make one attack, it fires 10 bullets.
If you aimed at one target with your one attack, you determine how many of the 10 bullets fired hit the target by how much you beat the target's AC.