You are now in the warhammer 40,000 universe! You are "you" not a marine or even a lowly guardsman. You are just yourself, transported to the grim dark future to fend for yourself and survive.
You have to arm yourself for protection, for hunting etc. What 40K weapon would you have to have to survive and flourish in the harsh future?
Here are the simple rules:
1) Weapon must be man portable, no titan weapons or vehicles. If you can haul it, you can have it. Pistol, rapid fire or even heavy weapons are ok. No, you cannot have "an assault cannon because then I would have terminator armor and that would be awesome!" type of crap. YOU have to be able to use it, haul it, maintain it and fire it. Not you in a suit of terminator armor or in a titan or any other vehicle, including dreadnoughts and walkers. Bikes are out too, sorry.
2) If you take a pistol, you may also have a secondary weapon that is a hand held close combat type piece of equipment like a chainsword or the like. Again, no "titan power claws cuz then I haz a titan yo!" It must be MAN PORTABLE!
3) Only current codex weapons. If it ain't current, you can't have it. No Kai guns for example.
4) explain your reasoning.
thats it, have at it! (let the rule bending begin... Yeah, I know you people well enough!