First, I miss multi-qoute. I am well aware that conformity stifling creativity is a major theme of the game, hence my use of the phrase.
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Secondly, I see there is a serious disconnect between my discussion of using the rules to do something and the belief that I am advocating for the same. The original poster asked is something was possible, I said maybe using AWM. My understanding of AWM was flawed and it isn't possible with the RAW. We then moved onto the usefulness of the AWM upgrade including how recoil might affect it. I offered that energy weapons would not have the same recoil problems that chemical projectiles would and used a specific example. Somehow the conversation was derailed from a discussion of rules into a discussion of why something would not happen in-game with a specific faction. I never said Space Marines should or would be walking around with dual shoulder mounted plasma guns (or dual shoulder mounted weapons of any kind).
Again slavish devotion to cannon limits your creativity. There is a difference between saying you cannot do X or Y would never happen because it would violate cannon and maintaining the verisimilitude of the fictional setting in question. Maintaining verisimilitude allows you the option of "violating" cannon as long as your reasoning for something is consistent with the internal logic of the fictional setting. Slavish devotion cannon does not allow you that freedom as you limit yourself to only what is currently written about the fictional setting, you cannot add to it beyond its existing strictures.
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Before I continue, note that I never stated that players or GM's should mount twin plasma guns on every suit of armor or that Space Marines should. Per your logic argument, there are plenty of reason not to, especially given the Imperium and Mechanicums' attitudes and limitations regarding technology. They include tradition, weight, and logistics. The Tau are able to do so through superior technology, utilizing a larger weapons platform and their attitude to toward innovation. See below for my comment on the "munchkin factor", the same applies towards your mechanics argument (though its a different fallacy).
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Argus Van Het,
Please take your red herring/ad hoc/false cause fallacies about munchkins somewhere else.