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I'd rather have them with bolters but also toting chainswords, just looks cooler that way.

Or you could attach the chain blades to the front of their bolters as assault attachments.

What You See Is What You Get, it has to be present on the model for it to be used in the game. That is the standing rule. There are some minor exceptions, the bolt pistol + ccw and grenades thing being one of them.

Cut pistol from hand and replace removed pistol with bolter. Presto! Marine with bolter and chain weapon in true grit style.

WYSIWYG should only represent options that are given to a unit/IC/Vehicle that can be modeled. If a Havoc is packing an Autocannon, he should be packing an Autocannon, but I see no reason why the Bolt Pistol and CCW should be on the model since there is NO ARGUMENT to wether or not he has it...I just show my opponent the codex and that's it.

That's my stance at least. WYSIWYG should only be for options clearly modeled if possible, not for basic wargear. I mean, look at the Dark Angels Company Master, the one with the Power Sword...He has a big fricken banner where the Iron Halo should be, but he has an Iron Halo in his war gear...what am I to do, chop the banner and give him an Iron Halo? No, it's part of his basic wargear package, and he HAS it whether or not I modeled him with it. For the record, the Company Master ALSO has Frag and Krak grenades, yet none of them are molded onto the model.

In your face, WYSIWYG!! xd

But that is defeating the purpose, I want the gun in the left hand, since the chainsword should be in the right.

Using loyalist parts (atleast arms) is a quite good way to convert chaos beakies.

I belive there's lefthanded pistols in the assult marine boxes, ready for stealing to do this little weapon swap.

Ask around friendly marine players for left-pistol hands, or maybe if you have access to buy stuff online you can get bits on the battlewagon website, since they sell loose parts.