Necramech wastes primary ammo while in 4

[SoulEater011]

While I'm using my Necramech's 4, Arquebex, my primary weapon continues to fire. This causes it to shoot and deal damage, but I don't think this was an intended game design. When I exit my 4 and pull out my primary archgun again, I see that I have consumed a considerable amount of ammo and need to wait for it to recharge. Sometimes, I'm at 0 ammo despite using my 4 the entire time. Thanks for reading this.