Halo 5 did worse than Gears 3.Yeah, and Halo 3/reach/4 did way more than Gears 3.
Halo 5, according to what we have for NPD
(October 27th release day)
October = 935k
Next Month = ~358k
Total = ~1.3 million
Gears of War 3
(September 20th release day)
September = 2.0 million.
In just 10 days, GeoW3 sold more than Halo 5 did in 30.
Gears 3 was on 360 and so were Halo 3-4. Halo 5 and Gears 4 are on XB1. Gears 4 has new devs too which is usually not a good sign for sales.
