Odds are you'll pick up plenty of ammo if you're using a SPAS, so you won't need ammo salvage. Stabilizer does not affect the Carbonizer, and isn't necessary for the SPAS, so I'd say don't bother. Explosive Focus is much more likely to help you.
Reigns of Terror aren't easy to get unless you're playing against newer players who have no idea what they're doing. So I can't make any suggestions aside from: don't miss.
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