Simple: For Fun, but likely to remind himself he used to be fully organic. A-17, a.k.a, Lapis (yes that was his name when he was human, his Sister, A-18 was named "Lazuli"), used a firearm when dealing with people, while A-18 was mainly the beam spammer.
Also, in the Good timeline, A-17 used a Rifle when he became a park ranger, likely for similar reasons he did so in the bad timeline, mainly to remind himself he was once fully human, but also for fun.
Simple: For Fun, but likely to remind himself he used to be fully organic. A-17, a.k.a, Lapis (yes t
