When do you ever use the plasma guns, seeing as you need to be in an Engage action to be able to firefight at 15cm, and through my current understanding of the rules, I'd rather firefight with an FF of 4+, than shoot with a AP5+/AT5+.

Am I missing something?

The main way is if you want to shoot with the Stormtroopers, laying a BM and possibly causing casualties, then Engage with another formation, allowing the Stormtroopers to support. There are also some formations that you just don't want to engage, but can do enough damage to through shooting alone to break or render ineffective.

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SG

Ghost's Paint Blog, where everything goes that isn't something else.

Three times I can see it being useful.

Overwatch against an expected enemy appearance (opponent has teleporters/planetfallers/etc).

Opponent is further than 30cm away. You can double, and still get shots in preparation for the next turn.

Opponent is likely to assault your formation, and you can't just give it up, but an assault against them is too risky. Remember, you fail an assault, you break. That can't happen if you just fire.

Also remember, if you aren't humming or whistling this whenever you activate them, there's something wrong with you.

Morgan Vening

If you are both contesting the same objective and your opponent is close to breaking. If you assault you take the chance of losing the assault, even if it is a small chance. Instead you shoot and break him in safety.

There are plenty of reasons to shoot and not assault.

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author of Syncing Forward and other stories...

It's a dog-eat-dog world, and I've got my Milkbone underwear on.

Also remember that individual companies are quite weak, but if you keep two companies within 20cm or so of each other, especially if they are in cover, then it is a very different story, because each can support the other. Keep other formations in close support; try to move as coherent 'commands'; use a 'reserve' to fill gaps left by broken formations; Use Sentinels and Rough riders as screens, but keep them close for mutual support; use artillery, Super-heavies as distant fire, and Leman Russ companies and platoons as spearheads etc

The great trick is to form a rolling wave, moving like a tortoise with formations moving in support and using cover.

Equally, as with most other races, upgrades will make a big difference - heavy weapons in the centre of the formation, putting them in chimeras or even adding Leman Russ platoons.