I went to Xur, saw he was selling "Cerebus+1" and was hyped to get it. When I clicked buy nothing happened, so I went to orbit and back, and he was suddenly selling a different weapon "Riskrunner"...
I ended up buying riskrunner, but am disappointed as I'd much rather have cerebus+1, and other people could still buy it even though I could not.
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You'll do way more damage with Riskrunner, tbh, in both PVP and PVP, although there's a quest to get it for free. I'm only using Cerberus atm because I've got a catalyst to complete for it.