Veteran
Hitting at least 3 enemies with an attack, increases your damage by 24-36% for 3 seconds.
2.5-4% Bonus Critical Chance on Chained Hit. Stacks 5 times.
Target receives 1-4 Stacks of 2.5% Brittleness on direct projectile hit. Debuff lasts for 5 seconds and can have a maximum of 16 stacks.
Ranged hits add 1-4 stacks of bleed to enemies.
This build works with your melee weapon of choice, although the 10% melee damage node taken just after grenades is specific to chainsword breakpoints and may be substituted if using a different melee weapon.
Trigger discipline is key to the bolter:
Of course sometimes the emperor (and the situation) demands a good mag dump :D
Some other points for consideration:
Praise the emperor and pass the ammunition!