I was reading through my 5ed rulebook for 40k and I have to say if they just renamed the current DT “eviscerator” to “heavy chainsword” it would be lore accurate immediately.
Chainsword - Strength of User with no AP value
Heavy Chainsword - 2 handed weapon with +2 Strenght and AP 5 (flak penetrating)
Eviscerator - 2 handed weapon with 2x strength and AP2 for power field
Then they could add a REAL eviscerator with the BIG SIZE.