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I see on my Sentinel box a depiction of a Sentinel with a heavy flamer and a chainsword (CCW?).
Yet I see no way to actually arm a Sentinel with a CCW (or more than one weapon). (In Army Builder, or the IG Codex)

Is this just decorative? Is there a rule somewhere else for it?

Look in the doctrines in the codex, I believe it's the Hardened Fighter that allows you to give the Sentinel the extra weapon/attack.

Otherwise it can be used to indicate a "rough terrain mod" (that's what catachans use them for, hacking through overgrowth.)

And they are geting a whole new bit of weapons in the new codex that comes out in a matter of months. Might be prudent to wait until then to assemble it.

I use the chainsaws to indicate a "rough terrain mod" like chromedog said otherwise i do not think that they count for anything.

If I'm not mistaken the sentinel chainsaw was created for the long since outdated Catachan Codex.
Catachan Deathworld Sentinel Squadrons all had an extra attack, WS 4 and always had rough terrain modifications.

So the chainsaw actually represent both Hardened fighters (+WS, +A) and Rough Terrain Modification.

catachan sentinel is awesome for tying up SM tac squads or pretty much any GEQ squad =)