With the Dark Heresy books currently available, what character options do I want to invest in to be as effective as I can be with a lasgun, in combat, as an Imperial Guardsman? Assume the build is targeting a Rank 4 character. I do not need a complete character build.
Increased WS. Max it out. Makes hitting and open ending both more common.
I can't get to my books, to look up the correct names, but...
There is a talent that converts aim to +20. It's well worth it.
There is a weapon specialist talent, as well - take it. IIRC, it's +10 WS.
I had a Rank 4 IG player in my DH game who was routinely having to-hit chances in excess of 100... which means not only hitting a lot, but almost always getting to open end on 10's on damage. (Remember, there is a minimum chance of failure... but it still improves quality of hit - vital for autofire)
Max starting WS is 40; the additional +20 from raises, and 10 from specialist gives a 70% before aim. Adding +40 from two improved aims brings that to 110, before situational modifiers. Autofire can bring that to 130 with the right talents...
Don't forget a high quality weapon, either, for a bonus to hit and/or damage.
If you don't miss, you maximize your damage.