You say i should stick to 120mm because of fluff, but also suggest more practical, custom designs - this doesn't really mix :P I considered doing LR 120mm and Basilisk 132mm (fluff caliber). However it just doesnt match their appearance and visuals, so i decided to increase their caliber to something that represents their visuals better (without going overboard). Because if you think about it - the weapon calibers are something they came up with out of the blue basically, they've never done any tests how it would really look or fit. I try to stay very close to what the background and so on gives me, but if it means completely changing appearance i will make compromises. It's a matter of balancing trueness to visuals and trueness to text-fluff. And imo i walk the middle ground. Let's not forget that GW would not blink an eye to shoot stuff down if they see harm with it /disparity with how they want it to look.Personally, I think you should keep things to true scale. Plus the Leman Russ has a 120mm cannon, not a 150mm cannon. Scifi weapons like lasguns could still have slightly larger/thicker barrels. 40k is heroic scale because of the miniatures, and in a game like arma there's no reason to stick to it. Expanding on your multiple pattern idea, I do like it and I think maybe for the most high-tech patterns, you could make them slightly more practical designs, maybe. :P Or add semi-fictional variants based off of conversions. The 40k RPG also has a ton of weapon variants you could look at.
Weaponvariants for the 40k RPG's are mostly handweapons - i have them all cataloged in my "reference archive", so yes, i consider them for PDF / Cultist usage. Though standard issue weapons are more of a priority for the near future.
I also know about the OFP team, but they have a different quality standard than i do and they also seem to have their own ideas regarding how things work. We'll see.