@xpl0its I think it's the Flatline, because lately it's been the same thing for me. If I am in a 1v1 with an opponent, we both fire at the same time, and I lose with the Flatline, while they win with virtually anything else. This is when it looks like we have the same body armor on. It's even worse with the Spitfire, but I think that's because its rate of fire is set so low.
I'm not even sure what the functional difference is anymore between heavy ammo and light ammo, because you would think the heavy ammo has a larger caliber and would travel at a higher velocity, both of which would equal more damage - but it does not appear to be the case at all.