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Dirtside 2 is a 6mm micro armor game for modern and sci-fi gaming. It's been out for nearly 20 years (currently OOP but like Epic, it's a free download).

It has optional rules for all sorts of things from drop troops to orbital bombardments to building your own vehicles. I like Epic (I still use Epic 40k) but I like DS2, too.

Here's links for the rules and an on-line fan created vehicle creator; I've used the vehicle creator to port my Space Marine units into DS2, so I get double duty from my minis.
Just thought I'd throw out another option for my fellow 6mm fans.

http://www.gzg.com/ --- go to downloads and the DS2 rules are there for free.
http://www.cowell.org/~andy/min/ds2/

Dirtside 2 is a great game. Chaos is the only epic army that doesn't translate very well.

Chaos will always be difficult as it really just Marines with pointy armour. My own feeling is that it would be the only way to reflect the "fantasy" elements of 40k a lot of abstract rules for the various factions.

That said generally, like adapting Stargrunt, it generally comes down to tinkering with the values and adding your own house rules. The system is generic and if you do not like something either do not use it or put something in its place.