Whilst all of this discussion is great, I think some folks are over thinking things.
From my point of view, I am a great believer in keeping things as true to life as possible - I think this makes for more realistic and believable whiffs. If we can learn anything from the history of AFVs, it is that history is fully of vehicles that were far from perfect. Indeed the M3 itself is a prime example of a tank whose severe imitations were well known even at the time of its introduction into service and yet enter service it did. Consequently, I am building my whiff in the sure and happy knowledge that its design is fraught with technical and tactical limitations. In other words, no slaved gun controls, limited traverse for said guns, a six-man crew (full marks to the elder Wombat) and a probable ammunition load-out that is not ideal.
In the end, this model will depict a whiff AFV and there is absolutely no law that says whiffs have to always represent AFV that were perfect in every way. Come to think of it, name me an AFV, any AFV, that is.
