1) Compability with Warhammer 40k - Genes and Psycasts - it adds chaos and loyal astartes factions, why them don't use this weapon?
2) the same - impact bolter can be used by astartes in power armor, not only warcasket. it's their iconic weapon!
3) storm bolter - for terminators (astartes in cataphract armor)
4) assault cannons, heavy and hurricane bolters for warcasket only - dreadnoughts are usually armed with the heaviest guns
5) and of course combat extended