I remember reading in the IH that autoguns are often used to train military units in the Imperium due to their high rate of fire. This allows recruits to actually hit something fairly frequently. Consequently it also says they are quite popular in PDF forces where the skill level is usually below par. Therefore its reasonable to suggest that a guardsman might have experience with autoguns and know which one hits the hardest.
Also the only reason guardsmen use lasguns is due to the battles they're fighting. These guys are expected to land on a hostile planet with little to no reinforcements or available supply lines. Then they have to secure a headpoint and fight the enemy until aid arrives which might take anything from weeks to years. Thus the lasgun with what ease it is supplied with ammunition is their best friend. Think of it as the AK-47 of the future only you can leave it out in the sun for a day and it'll have a whole clip of ammo the next.
-- I think they're fine), I would take into account that they are lasers, beams of light that move at c. They have near-zero time to impact and are unaffected bt gravity, wind conditions, etc. (except for dusk and smoke). You point the barrel at something, and it will hit that point.