Originally Posted by
CrazyFingers
That is correct, the 15-22 upper and lower are slightly different in size and the pivot/take down pins are spaced slightly differently, to prevent someone putting a 5.56 upper on it and blowing the 15-22 lower to smithereens. Since the 15-22 is a blowback .22, it has a polymer fake receiver extension that has a molded solid end where the buffer would go. Imagine what would happen if you could put a real 5.56 upper on it and pulled the trigger. However, you can use all AR-15 mil-spec stocks, pistol grips, and even triggers in the 15-22, as those parts are the same size and spec.
The Smith & Wesson M&P 15-22 is definitely the best dedicated AR-pattern .22 rifle (in my opinion). It was designed from the start as a .22 AR, and therefor avoids some of the issues that .22 conversions and .22 uppers have.
First, if you use a drop-in .22 conversion in a standard AR, your accuracy is going to be terrible. Firing a 36-40gr .22 LR round through a 1/7 twist 5.56 barrel means you'll be doing good to get a shotgun-like pattern at 25m. The 15-22 has the proper 1/15 twist for a .22LR projectile. This leaves a dedicated .22LR upper for a standard AR lower. From what I've read, these can require significant fiddling to run reliably, and additional parts if you want the bolt to lock back after the last shot or if you want the bolt release to work and be more than a decorative piece. If you search, you can also find a 15-22 for close to the price of a dedicated .22 upper, and you will then have 2 fully functional rifles, instead of 1.5 rifles. This also avoids .22LR gunk from coating the internals of your 5.56 lower.
This is why I bought a 15-22 to go with my BCM. Personally, I believe it was the best choice for functionality and economy.