The forearms interchange, I put on a SWISS SIG 551 Green forearm, changing all the furniture to green. Its actually alot easier on the 522, the Gas Block Front Site is held in place by 1 Allen Screw, Loosen it, slide the assembly forward, then the handguards snap together really easy, then slide the assembly back and give it a little tap, that will snug up the handguard, tighten the allen screw and reassemble the upper and lower, NO SLOP in the Handguards at all !!! Unlike the 556, where you have to shim it sometimes.
As for the Top Receiver Rail, the 522 is superior to the 556. The 522 Rail and Receiver is all one piece ! whereas the 556 is screwed on, there have been some board members that have had the screw studs welds break.
The only drawback is that the 522 does not have the built in emergency rear site, just solid rail.
Another tidbit, the 522 has a NORGON Style AMBI Mag Latch ! the 556 doesn't. The Lower Receiver is Polymer/Plastic lowering the weight, and the Trigger Guard is molded in, so there is no changing it out. I changed out the pistol grip to a Green 556 grip.
The Front Gas Key is actually a Plastic Plug, pull forward and it comes off, revealing a storage tube, cigar size for spares, bore snake, batteries, ammo, etc !
Aurora Front & Rear Sites makes it look like a Swiss Military Battle Rifle !
556 Bayonet Lugs work, but you have to change out the AR A-1 style flashhider to something alittle bit longer, such as a Vortex, Phantom, or SIG SWAT Flashhider. This is due to the 522 Barrel being shorter than the 556. The Flashhider thread pattern is the same as the 556 or AR.
Only other thing I might do, is change the Push Pins to the MFI SWISS SIG Style Captured Push Pins. Then it will really have the SWISS Battle Rifle Look, I found a SWISS Military SIG Sling that I put on it.
I keep getting load errors with the Vista and AOL updates, otherwise I'd post a few pics of my 522 with 551/556/PE-90 Parts installed.