A lot of the time, my 28mm bitz box. While I mentioned it being a nuisance in the other thread, GW's heroic style means that weapons and sometimes other things like hands are often so exaggerated in size as to be suitable for realistic 54mm miniatures.
The squad of "Cadians" I've put together use Lasguns from the Shock Trooper box, just with lengthened stocks (usually the ones left over after cutting away the original Lee-Enfield rifles, but sometimes plasticard instead). With the longer butt, they proportion well and measure up about 30mm long, proportionally about three feet long, a common length for a real world assault rifle.
Similarly, Sister Phoenix's Godwyn-Deaz bolter is a lightly converted Forge World Phobos pattern.
Even on heroic 54mm miniatures, 28mm rifles and heavy weapons can often be converted quite adequately into their smaller variants... or sometimes other options. I've seen bolters with their magazines and muzzle brakes cut off, which makes quite a nice stubber in the hands of a bulky 54mm miniature.
Sgt. Gillmore has a hellpistol that's a cut down lasgun, Inquisitor Skoll's (his new model, not the old one) stubber is a cut-down SMG from Heresy Minatures, and so on and so forth.
You can get particularly creative with combining parts, if you want:
This is Frost's coil-rifle*, made from variously lasgun, bolter and plasma gun pieces - although it is a little less detailed than I originally designed it
, which was mostly a question of whether I could make something look distinctly Imperial yet also unfamiliar.*Hence the appearance of two magazines. It fires solid projectiles (from the rear magazine) using electromagnetic fields (powered by the lasgun cell).