Plasma pistol is just too weak, with too low range.
Its not weak, I have a char with 14 AP and BRDx2, shooting 7 times with a plasma pistol per turn (TB build obviously)
All the way to metal works like wonders, you can easily shoot for more than 30 dmg on metal, however its utterly useless vs a BA or CAMK2, doing around 12 - 18 dmg a shot, but even then thats not really the bigger issue... The problem here is the range, 20 range is too low to make it a practical gun, more so than sucking vs a BA. The Fast shot build is ok for crafters and pure PVE builds but it really sucks on PVP, if they were to lower the BA absurdly high resistance on plasma damage then maybe THEN it could work.
Oh and just try using it on RT, by the time you pop your 7 shots, a guy with a LSW will have emptied its magazine on you