Had a weird scenario come up this afternoon:

My Shadowsword plus commissar was attacked by a squad of Assault Marines, who won the assault. Due to the Shadowsword being fearless (from the commissar), and being surrounded by infantry on all four sides, I elected to leave it in base-to-base contact with the marines.

At the very beginning of the turn, my opponent wanted to use their Terminators to move 15 cms and engage my Shadowsword using their firefight values. The Assault Marines can lend supporting fire by being 15 cms of both the Terminators and the Shadowsword, but the rules (even the GW update) state they can only use their Firefight value. We went ahead and played that the Assault Marines could only use their Firefight to support, but it seemed a little odd that base-to-base units could only contribute firefight. I know it's an uncommon scenario, but did we play that right?

Very intruiging indeed! I have never heard of this happening before

You played the initial assault quite correctly; being Fearless the Broken Shadowsword does not need to move and I assume this resulted in all 4x Assault marines remaining in contact.

1. Your opponent could have moved 2x Terminators into B-B as the Shadowsword is a DC3 War Engine, but evidently did not for some reason.
2. For 'supporting fire' I would actually allow the Assault Marines to use CC values as they are actually in B-B.
   Although the rules state that supporting formations may use their FF values, this is because the rules do not anticipate this particular case; usually other formations not actively taking part in the assault are are not in B-B

Definitely one for the FAQ - though in this case it is definitely *not* "frequent"

Terminators in B-t-B would have been ideal for the SM player, but they were more than 15 cms away; the Engage barely got them into Firefight range. The Engage choices were:

1) Engage with the Assault Marines, and the Terminators don't factor in to the equation at all by being more than 15 cm away, or
2) Engage with the Terminators, making it into Firefight range only, and have the Assault Marines support in some fashion

We elected to play rules-as-written, but it didn't quite feel right. It doubly-sucked when the Terminators managed to whiff on all of their attacks, and the Shadowsword managed to kill one and win the assault roll by *4*.

A weaker FF is probably the terminators main "weakness" - they only get 4x dice instead of 8-9.

The other possible options are:-

3) Move the Termies into 15cm of the Shadowsword and fire (firing is not a Terminator weakness ). Even if they double, this has the potential both of scoring futher hit(s) on the WE and providing support fire for a re-hash of the Assault marine fight, giving you a better chance of killing it the second time around (and incidently completely avoids the question of the Assault troops 'supporting' )

4) Ignore the beast altogether as it is Broken; the assault marines can easily go elsewhere and potentially be more usefull (though this could well be less satisfying as the beast definitely should have died 1st time round).

Yeah, those are all good options, this was more of a "learning experience" for the other player Strategy and tactics aside, we just wanted to make sure that the Assault Marines couldn't use their CC with supporting fire instead of FF.

As I said earlier, IMHO I would have allowed the Marines to use their CC values (despite the wording in the rules), because they were in B-B and the rules were not written with this, very exceptional, possibility in mind.

I'd have allowed the CC too.

Fearless units remaining where they are when broken (Especially in BtB) is one of the worst and most easily abusable rules in the game for me.

It's all subjective though. I'd say it's down to you and your opponent to decide during the game.

If the Assault Marines were in base to base with the Shadowsword when it was assaluted they should have been dragged into the assault and become part of the engage action (as if dragged in by a countercharge).

They then should use their CC attacks and count for outnumbering etc.

I agree with dptdexys. They would be directly engaged as if counter charged and contacted base to base.

Chris and myself actually had this come up in a game recently and that's exactly what we did.

Turns out that this was a mistake on my part - stupid Fearless Khorne assault engines. :/

Still, was a great game and it's something to learn not to do for next time.