Depends on the weapon. In lore terms, if you're carrying something like a lascannon or a multimelta, you'd drop it in order to pull out your combat knife if you were engaged in melee. Then pick it up afterwards, mutter a prayer to the weapon's machine spirit, and hope it fires as normal. If you're carrying a weapon that's integrated into your armour, like the Heavy Bolter's ammunition feed or Lightning Claws (or their variants, for non-Devastators in the same position), its not going anywhere. In practice, you'd probably clout someone round the face with it* and then go to the prayer muttering stage of the previous paragraph. *Apparently, Lightning Claws were designed with this in mind...who knew!?
I would be in favor of seeing some tactical harnesses, honestly. I am unaware of how server/client heavy this could make it though.
I recall several references in the novels to "mag-locking"/"mag-locked" weaponry. It'd be nice to see all Armaments but could get a little cluttered also some weapons mean the largest secondary you can carry is a knife and pistol just look at Havocs/Devastators. I can't see them fitting a Heavy Bolter and Autocannon on their backs.
I severely doubt they will be magically stuck to their backs because of their sheer size. Slings exist, if they're made of leather, chains, flayed skin, they will probably be used to let the weapon hang freely to allow the users hands to be free. Slings are used today in modern military forces. Example: Fullsize: http://fronttowardsgamer.com/wp-content/uploads/2011-05-01_00005.jpg The Ork Mekboy in DOW2 is a good example. His big shooter is slung over one shoulder, one hand pulls the trigger and the other feeds in the ammunition. He could let go of his gun completely to get his choppa out and he and his shoota would be fine.
I belive that this guy who made this threads major problem is that he wants a spacemarine rocket luncher as a secondary, when it in books and board game, is a specialist main at best you carry a bolt pistol and combat knife with that. And that other guy who said what if a devestator is gonna use his bolt pistol. A devestator is not gonna use bolt pistol while carrying a hravy weapon around. And stuff is maglocked on spacemarine armor.
Here we go you see; I wasn't just dreaming it! Power Armour mag locks, Orks will just chain things to their body and Eldar will....Eldar will...let's deal with them later.
Eldar are space elves that use teleportation to different planes of existance or some shit, who cares about them?
If im understanding the lexicanum correctly a tactical marine can be armed with both the boltgun and a heavy weapon such as Flamers, Meltaguns,Plasma Guns, Heavy Bolters, Lascannons, Missile Launchers, Multi-meltas, and Plasma Cannons. That would mean that a boltgun as a secondary would need to be accounted for, I don't think the tacticals use the larger backpacks so it may be practical to have them across the back. As for devestators would they be able to hold the heavy weapon one handed while using the bolt pistol with there other hand? Edit: do the eldar use secondary weapons on units carrying 2 handed weapons? Just skimming the lexicanum and not seeing any mention of pistols used by the dire reapers and fire dragons.
