The cheap route is the 22 conversion kit, accuracy will be limited, due to the conversion kits super short barrel inserted into the AR barrel, possibilities include degrading the AR Barrels rifling due to slug bounce, it would actually take a tremendous amount of rounds to actually do this, but the BIG thing is .22 LR is an extremely dirty round, lots of powder residue. Which means lots of cleaning , especially the gas tube and gas port on he front site housing. You could clog this from firing .22's and then the AR would jam when using .223 ammo, if not cleaned properly. I know, I've had several different conversion kits. If you like the AR style, than either a Dedicated .22LR AR Upper or AR .22 look alike. If you would like to have a SIG in your collection, go for the SIG. I just purchased one right at $400, but there were other board members that got them for $365. A gunshop owner told me that they get deals from SIG on the 522 if they buy 556s.
The 522 is awesome, super easy to clean, almost like an AR, push the pivot pins out , release the upper and the bolt carrier assembly slides out ! Uses the same furniture as the SIG 551/556. I changed out the forearm to a SIG 551 Green forearm and the pistol grip to a Green SIG 556 pistol grip, now I got to wait til Colorado Gun Works gets more PE-90 Green stocks in, to change out the buttstock. Also the Gas Regulator under the Front site is FAKE, you can actually pull straight forward and it will come off, revealing a approx. 6" hollow tub/cigar size, big enough to store spare parts, ammo, batteries, bore snake, etc. The Upper Receiver has the Picatinney Rail integral to the upper, which is what they should have done to the 556. But it does not have the built in flip up rear site as the 556. I put a set of Aurora Front and Rear SItes on, looks like the SWISS SIG Battle Rifle. Also it has a version of the NORGON Ambi Mag release on it, why the 556 does not have it, is clueless ! A Bayonet Lug can be mounted, but requires a longer Flashhider to mount a bayonet, has a AR A-1 style birdcage flashhider, need a Vortex, Phantom, or something similiar in length to hold a bayonet. Front Site/Gas Block is held on by a Allen screw, loosen this and slide it forward, and it makes snapping the Forearm together alot easier, plus as a added BONUS, when you push the assembly back against the forearm, it takes up the sloppiness and makes the forearm rock solid. The Lower is POLYMER with the trigger guard molded in, Pistol Grips can be changed with other SIG 55X series grips. I haven't got to shoot it yet, but it is supposedly alot more accurate than the UMAREX COLT .22 AR Clones. SIG 522 also uses standard Black Dog Machine AR .22LR MAGS !!!
I'm a fan of both the AR and SIG, once I sell off some parts and guns, I'll get a Dedicated .22LR M4 upper to go along with my SIG 522 in my 22 military style collection .