"I bought my ARP 556 off AirsoftGi about a month ago, and here's my review. First, the battery space is tight as most buffer tubes are. I bought a li-po battery from hobby king, dimensions are 101x20x11mm. Still tight, but doesn't go out the buffer tube. In fact, probably could fit a slightly larger battery if necessary. Stock has very little wobble for a PDW design, to the point where I don't mind. it's a bit small to shoulder, but the trade off is fine, because it's very compact and feels solid.
The barrel feels nice to hold, the edges are nice and smooth, and weight distribution is good. If you don't like the muzzle decoration, don't count on changing it because it's on really tight, and I have no idea on what tool it would need (not that I want to change it, but...). I put on Magpul Rail covers, and it feels sweet.
Mag compatibility: fits PTS EPMs but can be tight, Ares mags as well. VFC mags don't work, and the included hi-cap mag is nice.
The gun fires nice, but very whiny. Need's shimming, but works fine even without work on the internals. Chronoed it on .28g bb's, and was about 350 fps consistently, but I couldn't get a very accurate chrono because the bb's somehow got mixed with .23 's. Either way, fine gun for $300.
I would recommend this gun for anyone starting their first gun. It's an m4, can be modified easily, and the Mosfet and ETU will extend it's life for a long time. If you're going to upgrade, it's solid body won't break before the gearbox does. I haven't opened it up, and don't plan to because besides the noise, the performance is great, so I plan to run it till it breaks and work from there.
If you plan to gut the gun and replace all the internals, I think that this gun isn't the one for that. If you want the aesthetics- then by all means, but otherwise spend the $300 on a cheaper gun. The included mosfet and etu are better off replaced with a more expensive unit anyway.
For those who are wondering if it can be used in outdoor games, I'd say yes it can. Obviously it's also usable in cqb, so it's an all-rounder.
Final thoughts: It's so cool! the M-Lok handguard is solid (in fact it's too solid and I can't take it off) and is very pretty. The gun is worth it. Also, as a bonus the selector comes without painting, so you can have fun painting it yourself, if that's something you like.
Side note, if you want to use a sling, there is not much on the stock to mount on. There's a hole at the base of the buffer tube that a hook could fit. I bought a HIT sling from Advanced Gunslinger Armament, with a hook and qd pin, and bought an M-Lok qd mount. Looking back, should of got a rail qd mount, because you only have 4 M-Lok slots, plus 2 on the bottom. If you want any side rail attachments for a tacticool build, like a cqb flash light/laser combo and/ or rail covers, get a rail qd mount (assuming you get one at all). Any sling mount should work (smaller hooks probably fit too), but it also looks like a part that might break under stress.
Oh, and the burst mode is great too. Want to shoot in volleys of 3 but can't manage the pull? Then the ARP556 is the gun to get (unless you can afford any mosfet that handles burst...)