Depends on where your unit is, and if you are using the MT or AM codex.
I like volleyguns in my MT allies, but for the units that are going to have a more defensive role. The Tempestor's order to twinlink guns is very good for stationary volleyguns (or the 4 special weapons in his unit). But if it is something that is ranging out or deep striking, meltas or plasma may be more appropriate. That said, if you are gonna be fighting power armor, it's hard to trump the volume of fire you get from a volleygun.
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