Bolter perks for me are the infa red type optics to see stealers in the scope and +damage barrels with a larger ammo clip and reliability
my favourite classes are Apo, Tactical and Librarian
With Apo and Tactical i have the MkI and MkII perforation enhancement which i think is +20% and tactical gets a faster melee speed as well
on Librarian i have gone with +30% resilience perk for now....
The armour boosts on these three are for extra ranged damage but i am playing around with ether +melee or +armour but for now the extra damage seems to work best
Bolters are tough when solo at No Mercy, well to be honest at my skill level nigh on impossible but i love a challenge but if i drop down to Lion (hard level) they work perfectly for me...just the right amount of challenge vs reward
Personally i do not like either plasma cannon and hell fire and flamer tend to block line of sight which is frustrating but bolters as support for Assault or a Heavy with an AC or Vengence provides a top team or a full squad with bolters only.... that for me is Space Hulk right there...
Bolters work best supporting the heavy guns for the long range snipping shots and softening up long range targets before the AC, Flamer, Redemption etc...can get to them effectively ... also in close quarters empty a clip into a stealers face will hold them back then you can finish them with melee.....and just hope there is someone else near to cover you while you re-load
So i see them as a 'support' weapon however saying that we've had some cracking 'bolter' only games on No Mercy
.... all in all Mk1 and MkII bolters are my favorite all purpose weapon