Bancrose you are forgetting the true role of the B18 spec, as pioneered by our lord and savior aoi bell, he's a heavy rifleman. Dump that nade launcher and fuck grenades in general. Grab that rifle and shotgun and go fucking ham on the attachments to make yourself the super robusto soldier marines are begging for.
Shitposting aside, Sadar spec is the only one that can reliably effect games with xeno blood raining down from the sky. The sniper is meh but can prod the boilers in the hopes that they with be shoo'd off. The scout is either shot by his own or gets murdered by a rav. Pyro is a coin flip, either it's FF galore or Kane's finest black hand soldier. B18 as above is the better build, because grenades in general are a hit and miss tool that often blows marines to bits.