So, at the end, Splatoon will outsell Halo 5. HANDS DOWN!

[PAOerfulone]

Well, now that we have first week numbers for Halo 5, let's compare them to Halos 3 & 4.

Halo 3 - 3,811,047
Halo 4 - 3,662,557
Halo 5 - 1,457,849

Halo 4 has been on the market for 156 weeks now, compared to Halo 3 both games sales were:
Halo 3 - 10,924,767 - First week numbers were 35% of that.
Halo 4 - 9,541,543 - First week numbers were 38% of that.

Now if Halo 5 has Halo 3's percentage effect, 155 weeks from now, it would have around 4,165,282 copies sold. If it has Halo 4's effect, it'll be at 3,836,444 copies sold.

And now looking at Halo 3 by itself, it is at week number 422 on the market, and it sits at 12,057,422 copies, and it's first week sell-through is around 31.6% of that number.
If Halo 5 has the same effect, it'll be at 4,613,446 million copies sold, 421 weeks from now...

Now let's look at Splatoon, which as of now, sits at 2,112,178 million copies sold...
And based off these numbers and projections... I do believe that Splatoon has a chance to finish with more than Halo 5.
Granted, it's not a big chance, but it's not a small chance either.
It has a reasonable chance. It's going to get a boost from this upcoming holiday season, granted Halo's will be much much bigger, since it was released right before the holiday season; And Japan especially loves this game, something that could never be said for Halo.
Will Splatoon actually do it? Only time will tell.
But I wouldn't dismiss the possibility.
I don't think it's a Guarantee! But it's not impossible either.