Well lets say at end of the eyar x1 has a 19 million userbase. 7,3million is a good amount. And like vivster said mose peopel who own the xbox one now are hardcore fans.

Halo reach and halo 4 are both over 10 million, I think halo 5 will be at 8.8 million by the end of the year.
The positive reactions to the online and the hype will bring the game close or pass halo 3 with sales when its all said and done.
People have to stop underestimating the halo series

8.8 million would be the fastest selling halo ever. 5 million by the end of the year would be ok, 6 would be great, and 7 million would be amazing if not a little to optimistic.

8.8 is way top much that would be like the fastest selling game of all time.

These predictions are redicilous and push the absolute limit of what is possible.   The main thing that is illogical about these sky high predictions is that they are expecting attach rates far beyond any other title in the series.   Guys, the best way to make a prdiction about sales is to look at past attach rates and apply it to the Xbox One's current (albeit unknown)  sales numbers.  Even if the Xbox One hits 16 million by year end, 8.8 million is still over 50% attachment.  Even if the Xbox One sells 17 milion by year end, the attach rate would still be over 50%.  

So you expect X1 to have a MASSIVE drop YOY this holiday? There's no chance that it will sell under 3 million during the holidays.

The last 2 Halos sold 9.75M and 9.5M copies LTD, down from a high of 12M (a 25% decrease) on systems with significantly larger install bases. I see H5 doing well, but selling 77% of the previous game's LTD sales, in 2 months, with a smaller install base...that just doesn't seem remotely reasonable. If the trend continues, we should expect it to sell ~6-7M lifetime, but I expect it to better that.

That doesn't make sense at all. Incuding digital there both over 10 million. And since 4 is newer it likely has a lot more digital sales, so it's almost certain that it outsold reach. If u don't want to count digital that's fine cause 4 is still outpacing reach. If the trend continues 5 should hit 10 million, I'd include digital tho cause it's so big now. 

As far as my guesses go, 3.1 million fw, 6 million by end of the year. Anything under 2.5 million fw and 5 million end of the year will be very disappointing. I attribute these lower numbers to the low install base, it will sell well over time more like 3 cause it's early in the gen and people will buy it as they purchase consoles. I highly doubt it will be front loaded like the later games in the series.

I honestly have no idea what the Xbox One will sell this holiday but I do expect a drop YoY  because the pricepoint isn't the biggest incentive for the Xbox One anymore.  So, I expect it to be behind all November except for Black Friday and the incredible high value $299 bundle.  That by itself will retard sales all month.   But for the sake of argument lets say that the Xbox One is at 15 million and it DOES sell 3 million this holiday.  Well that would put it at 18 million.  

8.8 million copies by the end of the year is still far beyone Halo 4's attach rate through December 31 (not to mention higher than Halo 4 as well)

VGChartz puts Halo 4 at 7.2 million in 2012.  The Xbox 360 had sold 75 million at that time. So roughly thats 10%.  So that means that the Xbox One would have to reach 80 million units by the end of the year if the attach rates hold.   Thats impossible.  So, when people predict 5 million (like myself) we are being incredibly optomistic.  

Don't get me wrong: I expect it to do well, but trends are trends, even if not wholly predictive. If we don't go on them, however, we're just throwing numbers against a wall. As another commenter said, those numbers indicate ~50% attach rate, which is simply unheard of, for Halo or any recent gen game. People need to add some reasonability to their expectations.