My friends and I just started playing Deathwatch and my first character is a Storm Wardens devastator and I had a question on wielding both a heavy bolter and a sacris claymore. Can you even fit the giant sword on your armor with the heavy bolter and the ammo backpack and if so, what is the actions required to switch them in combat like a half action to "holster" the heavy bolter and another half action to draw the sacris claymore

Correct on the actions. Space marines may Ready a Basic weapon or any melee weapon which can be wielded in one hand as a Free Action because they have the Quickdraw talent. The Heavy Bolter and Sacris Claymore are neither of these therefore it costs a Half Action to Ready both (take out and put up).

As for room to carry it, I'd probably have a large scabard under a non-shooting shoulder, the handle of the blade extending over the shoulder behind the head. Maybe even have the scabard attached to the side of the ammo backpack. Don't expect to be carrying any other weapons on slings though, that would be pushing it beyond the limits.

herichimo said:

You can always opt to NOT take the backpack ammo feed for your heavy weapon and just go with what clip it has plus any reloads you can afford to buy with requisition. If anything this makes you MORE flexible by sacrificing how often you have to reload in exchange for ability to carry more gear.