Varnias Tybalt said:
Flak armour as they are described in 40K seem to be of rigid design, but still able to protect the user from solid projectile weapons and even certain types of energy weapons.
"secondary sources like the infantryman's uplifting primer"
1) Well... if we suspend our disbelief and say the 40k universe is real it would actually fall into the primary, rather than secondary, source category.
2) It is written as a piece of propaganda... very little said in the Primer can be taken as read. Otherwise hammerhead railguns are as powerful as hotshot lasguns...
Heck, way back when flak was as protective as chainmail against all weapons (meaning pretty much not at all except against needle guns, bows and crossbows). It just weighed less and could be worn with other armours. Of course that was a long time ago, and stuff has changed, but flak has always been pants.