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I understand that Plutonium tries and succeeds at replicating the original black ops 2 experience however I don't think adding this would impact it too much. I find it odd that this attachment wasn't fleshed out for the whole category as the DSR already has an iron sight model in zombies, but the SVU and XPR do not. The SVU already has folded iron sights on its model and the XPR could just share the sights used on the Ballista or another weapon, as I believe the Ballista shares irons with the KSG. It doesn't seem like too much modelling effort so why not complete the whole category too?
The iron sight ballista is rarely used, only by people who want a unique and fun sniping challenge and with the ballista being the most aggressive sniper I wouldn't see this impacting ballance at all. The only sniper I could see potentially being a problem would be the SVU but I feel that weapon would fall short to the FAL and even the SMR due to the lower firecap and handling, but feature 1 shot potential.
This is something I've always felt should've been in the game and would love to be able to use it.