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The Mass Combat rules under 'The Detailed Method' on pg. 292 of the core book do not mention what the Ballistic Skill or Weapon Skill would be. One could assume these would be the average of combatants in the unit, which makes perfect sense. However, the rules for hiring soldiers on pg.271-272 do not take into account what their Ballistic or Weapon Skill would be.
Additionally it does not specifically provide rules for arming any combatants hired, unless one makes the assumption that the 'Combining Acquisition' rule on pg.271 applies to mercenaries as well as items, or that the equipment is purchased as a series of separate acquisitions.