Of course, there will be the obvious issue of weapon balancing, but can it be done? Adding new weapons and ammo types for them throughout the Black Mesa campaign? Since we're dealing with marines, some sort of M4 or M16 variant seems like a natural fit. The niche I could see it occupy is that of a more powerful automatic combat rifle with rarer ammunition. Maybe a semi and burst mode for the primary and secondary fire, respectively. Or only burst but with a scope mode, similar to the revolver's ironsights or the crossbow's optic.
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