With the release of Season 2, 343i updated the Loot Caves on Fragmentation to now contain the upgrade campaign variants that could be acquired by killing HVTs or upgrading FOBs. These weapons were very powerful and fun to use, however, their ammo was limited because (for example) an Arcane Sentinel Beam didn’t technically use the same ammo as a regular Sentinel Beam. Unfortunately, I haven’t gotten a game on Fragmentation to see for myself yet.
Can the HTV weapons be used to complete weapon-specific challenges?
If 343 considers them different enough from their lower-tier counterparts that they can’t even share ammo, then it is reasonable to assume that they can’t progress challenges either. I don’t have any weapon challenges this week, so I can’t test this out myself.