Injection Rifle seems kinda bad. It has a lot of downsides and is very M.A.D. in comparison to other weapons. In exchange for Kickback, Volley (30ft), and Unwieldy, you get a gun with 100ft range, injection, breakdown, and 1d8 poison damage.
That very last one, I'd argue is a huge downside because of how many creatures are straight up immune to poison.
Personally, I'd either replace Breakdown or Unwieldy (because using it as intended, you're only getting one shot a turn anyways) with Versatile Piercing. You're shooting needles, I think it's fair to shoot 'blanks'.