So, I haven't been here in a while. But, I must return for this. See, when I left, we had a rule, right? It was the rule that stupid military technology cannot be used in the Halo era. Yes, you know what I mean. The AR15 series of rifles are not allowed here. Do you know why? It's because they're old. Really old. I mean hella old. On top of that, they suck. So I'm sorry, but you might wanna scrap this for something else.
Is it bad I'm trying badly to stop laughing? I haven't lol'd this much since K4's 'SUPER M4' or that Sherman tank (2552 edition)
But seriously, not-canon friendly/unrealistic for the following reasons
Who's going to call their weapon the 'apocalypse', take a look at how guns are named in reality
The only thing I can find that links to Kinetic Rail is train companies. I assume you mean rail gun tech? Wouldn't work for many, numerous reasons (and begs the question, if its a rail gun, why would it have any link to the M4?)
M4A1 in use from 1994 to 2117? Considering the US Marine Corp is actively (though subversively) beginning replacement of the M4 with the HK 416, SOCOM have, for the most part, replaced it with the HK 416 or SCAR and in the process giving Congress a massive middle finger. Imagining the US armed forces using it past 2020 ispure fantasy.
The M4 became a part of the US flag because it killed a guy on the most wanted list? For one, why on earth would they do this, and two, why would congress even allow this to pass? And then every flag in the UN? In case you aren't fully aware of how the UN works, its not some organization, the US is not some kind of over boss of the UN. The UN is, mostly, intended as a forum for discussion between nations and as a body for action (military, humanitarian, political, etc). In actual fact, the majority of power in the UN is held by European countries. I can't imagine every nation that has a representative in the UN (IE, almost all countries in the world) suddenly adding M4s to its flag for killing a beardy guy in a cave (especially since a few countries that deal through the UN may actually treat him as a hero)
Colt does not, in anyway, own the rights to manufacture the XM8. That is owned by H&K, who own enough industrial power, stateside and beyond, to manufacture it perfectly by itself. And considering in every weapon test the US Army or Corp has held between the HK 416, FN SCAR, Colt M4 and HK XM8, the XM8 has always come up on top, with lighter weight, requiring less cleaning, more infrequently and being vastly more reliable than all of the prementioned guns.) In act, the XM8 had 127 stoppages in 60,000 total rounds while the M4 carbine had 882.
And after all that, Colt just sold the rights to one of their highest grossing products. Never mind civilian market, foreign sales, after market modifications, market support, they just sold the multi-million dollar rights to some random guy.
This brings the question of why reinvent it in 500 years. The UNSC have much more accurate weapons, with higher calibre munitions, for reduced weight and increased barrel length. Basically superior guns in every sense and way. Why would this gun, a under powered, oversized weakling that would probably prove to be incredibly unreliable, as is its predecessor, even be looked at by the UNSC, or any self respecting military?