I was trying to build a melee chainsword character.
I think the best role is heavy gunner. It works if you have a regiment that grants BS as an aptitude so that you can take the Weapon Skill aptitude for your duplicate. Else, you could ask your GM to allow you to change BS to WS aptitude. You would also need to either requisition a chainsword or just ask your GM for one instead of your standard heavy weapon (it's lower rarity anyways).
Given Weapon Skill from another source, heavy gunner seems like the best matchup for chainsword use. You get a toughness bonus and the toughness aptitude (one of the most important things for a melee build). You also get the offense and defense aptitudes. You can take things like counter attack, true grit, etc that make melee builds awesome, and you can afford to level up your strength, toughness and weapon skill because of your aptitudes.
I was trying to build a chainsword specialist with commissar and ministorum priest, but I just couldn't afford any talents because of missing aptitudes. Offense and defense seem very rare and I couldn't find anyone with offense and weapon skill (except ogryns). I'm only using the base book of course.
I also take a shield (very easy to find/req)
I was wondering if anyone had a different take on chainsword builds or sees something I'm missing.