Ideally, I wouldn't ever suggest wearing pep or molded armor to an airsoft game. If you're using $30 walmart plinker guns that cant shoot for crap, maybe, but the higher end guns will slowly but surely eat away at the armor, thus wasting money and efforts. (If not quickly, lol)
A cheap gun from Walmart lacks the internal structure of an expensive airsoft gun ($200-$300) and is usually just tiny pieces held in place by screws and plastic poles. Without a mechbox its extremely hard. Mag's feed directly into the guns hopup, which is where the bb is fired from, Halo weapons utilize the very back of the gun for mags, so the size of an inner barrel would be outrageous, it would need to stretch from the very back, to almost the muzzle break, you'd have to physically make your own mag, or build a case for an already existing one, since mags feed into the gun. In theory, it seems possible, but in fact its extremely hard to do with a cheap gun. An expensive gun would be slightly easier because it uses a mechbox, and you can simply spend $70 on an outrageously large inner barrel, but you'd end up working off of a gun similar to the L85A1, or another rear fed gun. now that I look at it though, even the L85 wouldn't work... properly.
(Unfortunately there is no walmart version of the L85, walmart tends to only sell the FAMAS and M4's)
Someday some company may just simply manufacture a halo line of airsoft guns. =P
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