Firing the great plasma weapon leaves Nephthys vulnerable. He's still not sure how he's lasted this long unscathed. At the very least, the partially vaporized missile launcher user as well as those surrounding him falling down will reduce the magnitude of resistance, even marginally so. The Heavy Bolter sweeps the area. A few shots going wide, but one managing to graze his shield, detonating harmlessly. Nephthys can't help but notice that even if he didn't have the forcefield, the bolter round would have missed regardless. Hellgun-wielding soldiers also take shots at him. One piercing his shield but doing little more than create another minor wound. Nothing that would adversely affect the marine in any way but enough for Father Nurgle to gift with infection. The second shot enjoys the same fate as the heavy bolter round, dissipating harmlessly into red light.
The worst case scenario seems to be playing out. Warning runes and the barrel venting out plumes of plasma are indicative of a plasma gun that's going to explode. Fortunately the Plague Marine knew to use the more refined variant of plasma that never overheats. However in it's stead the intense heat loosens corroded components. Ducking totally behind cover, the Marine attempts the rites to clear his weapon. Which generally involves knocking the weapon around until he's confident the parts are back in place.
Spoiler
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Nephthys is totally behind cover now
Ballistics test to clear up jam target 51
(1d100)[47]
And Force Field test against heavy bolter shot
(1d100)[14]