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In GK army lists I always see Psyrifle Dreads

Dreadnought-135
2x twin-linked autocannon
Psybolt ammo

Why not take

Dreadnought-135
Plasma cannon
twin-linked autocannon

The plasma cannon is AP2 S7 it kills tanks and troops just as well as the cannon but has higher AP for the same points.
Why not take the Plasma

Different range
Not twin linked
Doesnt kill tanks as well as 2 S8 TL shots. NO blast weapon currently kills tanks particularly well, because of the scatter to half strength issues.
Lacks focus, as killing terminators isnt tricky ina GK army. Just pile wounds on them with S6 weapons

The autocannons also do great for mowing down Ork Nobz or Tyranid Warriors if there are no lightish vehicles to shoot at.

Because honestly all GK units already have plenty of anti elite infantry equipment. What GK infantry lack is long range anti tank firepower.

Plasma cannons suck at taking down tanks with decent armor. 4 TL S8 shots are very good for their price

S8 also ID's anything with Toughness 4, and would also deny FNP on most units.

Because you already have plenty of 24" firepower, and you need something longer-ranged to balance it out.