Yeah, you'd calculate the damage characteristic for the weapon then turn it into Mortal Wounds equivalent, rather than stacking the two together or whatever.
The Beastskewer could be good against monsters depending on the rest of its stats and ROF, but I fear it risks being a damp squib if it's subject to the RNG of just having a single shot, and if it just has this rule, it'll struggle to do much against regular infantry despite its flavour text of punching through the ranks.