Looking at my slide stop, it does have a slight bit of side-to-side play. I've also noticed that all of my magazines' feed lips have a bit of wear from the slide stop actually scraping the outside of the feed lips upon insertion. Might detail strip my XDM to see if there's any way for me to tighten down the slide stop. If I can't figure it out, I'm probably going to sell this thing.
I've picked up a 4.5 recently-ish and have noticed one really annoying thing. 90% of the time I insert a magazine, the slide drops forward without me touching the slide stop in any way. Anyone else encounter something like that?
H&K offers them with different grips now:
http://www.hkparts.net/shop/pc/HK-416-MR556-MR762-Battle-Grip-W-Storage-134p33.htm
http://www.hkparts.net/shop/pc/HK-416-HK-417-Battle-Grip-Variant-1-134p1023.htm
http://www.hkparts.net/shop/pc/HK-416-MR556-HK-417-Battle-Grip-Variant-2-134p1912.htm
http://www.hkparts.net/shop/pc/Vintage-HK-416-Battle-Grip-134p2300.htm
I think it might be one with some kind of folding stock, since the rear of the sling seems to be connected to the rear of the receiver rather than the stock. Probably some kind of AIMS or other AKM with a folding stock, slant muzzle brake, and Ultimak rail.
Look's like a LaRue 9". So you're right in saying that. I know for sure that WGC sells King Arms LaRue replica rails.
Edit: Whan is right about the whole 11" gooseneck part.
http://www.ebairsoft.com/handguard-sbrs-with-tube-p-2173.html this seems to be the only replica of it I've seen. It's ACM so it may not be the best...