Hugely strong magnets work - in one direction. But the instant you apply too much sideways pressure they will slip. Also, if you wear them too close for too long they can cause health problems. (Actually have health benefits, too, apparently. Ask my gramma.)
Straps are awesome, but obvious, and unless you're doing an ODST or Marine build, look out of place.
You can design a holster, which
PaiganBoi did for his ODST build, or you can permanently attach the prop to your suit. I've been tempted to do that a couple of times, actually. But don't think I will..... who knows.
Also, I found this video some years ago about strapping an assault rifle to a tacvest, and it's #*GENIUS!!!* I DO want to implement this technique into my armor suits, because, although there is a strap, it isn't nearly so obvious or distracting as it could be.
So basically for the assault rifle strap this is what I'm thinking:
Take your chest and back armor and put it on. Hold your assault rifle where it belongs (impossible in Halo armor, but do your best). Are you right handed? Or left handed? Once you know where you hold the rifle the most, take a length of strap. Stretchy strap will work great for our costuming needs. You will need to attach (a quick attach is best, but do what you can) the strap to the rifle at the appropriate place (I prefer near the rear of the handle). Then run the single strap from the rifle directly over your shoulder. Don't keep it too tight. Next, switch from your strong-hand to your off-hand and make sure there's enough strap for that. Mark where it touches the armor and cut it off a foot or so beyond that point if you've got it. Next, remove the armor and focus on the back. Where the strap reached the top of your armor, and on your strong-hand side, pull the excess strap straight down and attach it to your armor, cutting of any excess.
I will try to get a diagram drawn up here in a second; even *I* am a little unclear about what's going on.
So by now you have one single strap that doesn't wind around your neck stretching from your back, over your shoulder and out through your "neck hole" to your weapon. This allows you to maintain control of your weapon, attached to your body, AND you can sling it carefully over your shoulder to your back.
This method is awesome, but will take a little experimenting with to find how much strap allows the weapon to go over your armored shoulder, but it shouldn't be too difficult.
I'll get a picture on here as soon as I get home.