As a player I think any Skill or Trait granting bonus to hit is going to ruin the game. Just an example with my character...Guardsman with BS 48 and Hecuter with Targeter. That means all my shooting is done with base BS 58. Now single shot with aim is at 68, same for semi-auto, full-auto is at 78. That is at standart shooting range 30-60m. I rarely get to situations beyond this range so I get no penalties for long or extreme range. On the contrary I often get into a close range battles so I get bonus +10, that means my shots are at 78 / 78 / 88. And I only took two BS advances so far, so I can improve this value for another +10 if I want...and I most certainly will at some point.
Now, say I would have a Skill or Talent to get another bonus to my BS...there will be no space for misses...all the rolling would be pointless.
IF you want to give something to players to improve their effectivness with some weapons I would advise giving them a Talent with something smaller than a bonus to hit. As a player I would really appreciate something like this
Talented (Las/SP/Plasma pistols/basic/heavy) - grants Reliable quality to the respective type of weapon and category. So for example PC would take SP pistols - when he uses SP pistols they get the Reliable quality. Because of his great knowledge of this kind of weapons he can apply better maintenance or just solve field problems and jams really fast with some nice tricks. If a weapon already has a reliable quality it cannot jam, unreliable quality will be canceled etc.
You can work so many Traits or Talents to make subtle changes like these...for example Las weapons can get +1 penetration because PC can better set the output of the weapon or whatever...
Or you can invent a new combat action that can be made with this kind of weapon - like two single shots from a pistol in one round for a full action...empty a Revolver in one round at close range with some penalty etc.
There are many much cooler ways of how to solve this matter than just +10 to BS...
Think about it
There are some very good reasons not to upgrade to certain weapons even if you have the talents. Boltgun ammunition costs a ridiculous amount. A Guardsman at rank 3 (for example) can afford exactly 4 rounds of boltgun ammunition a month. Most of the Dark Heresy boltguns also don't some with full-auto (and seeing as a heavy bolter fires 160 thrones worth of stuff every burst it wouldn't exactly be very appealing even if they did). If you don't hand out ammunition or money freely then Bolt weapons are going to be incredibly unpopular, except to those that really just want to have one for the cool/intimidation factor.
Plasma and Melta weapons don't have this issue, but they do tend to have poor firing modes (melta weapons are single shot only, and plasma are largely capped at Semi-auto), tiny ammunition capacities, and large reload times. Due to their underselling by the Dark Heresy stats they are also not that great, unless you really need to toast heavily armed things. Plasma also obviously has the Overload issue, and most have Recharge as well. They just aren't worth the bother unless the target is simply too hard to deal with conventionally.
90% of the time the characters will be not much worse off with las and SP weapons, and SP weapons are the most commonly issued with Full-Auto capability and have alternative ammunition. Las is less appealing (which is a shame) due to full-auto being generally limited to specialist pieces, and the fact you don't have to buy ammo after having a couple of clips (as long as you have the ability to recharge the packs) isn't so overwhelming because SP is so cheap (unless you are really short on money, a Scum say, and burn through ammo quickly).
There is more of an "upgrade" issue with melee weapons, as there generally is no reason to keep weaker weapons once you get and can use the better ones. Ok, there may be personal choice, or maybe the campaign/mission style demands a little bit more subtlety than a chain or power weapon can muster, but other than that, Power beats Chain beats Mono beats Primitive. Shock... well, depends on your intentions, but will fall about the same level as Chain.
I think a lot of the discussion about having skills or Talented[weapon type] ignore that there are already plenty of ways in game to differentiate someone who has never picked up a gun before (the -30 untrained penalty), to someone who has basic competency with a weapon (Weapon Proficiency), to skilled marksmen, snipers and automatic weapon masters (the Marskman, Ambidextrous, Bulging Biceps, Deadeye Shot, Gunslinger, Hip Shooting, Mighty Shot, Sharpshooter, Quick Draw, Rapid Reload, and Target Selection talents all cover this).
There is plenty already in the system to differentiate between skill levels with weaponry beyond the basic differences in Ws and BS, i just think maybe some players and GMs haven't looked at it this way.
"Only the insane have the strength enough to prosper. Only those who prosper may truly judge what is sane."