I see people selling full on .22LR uppers for around 300-400 used.  Conversions howerver are roughly 140 brand new.  Whats the negative of goign with just the conversion kit?  

My main concern is if you fire a .22LR upper through a .223 chamber, is there damage that can occur to your barrel?  

Any comments are appreciated.

My dad bought a Colt Conversion, basically a magazine and bolt assembly.  I think he paid $90.00 in the blister package.  We have had it in his Non-Colt AR and my SP1.  Haven't had a single problem.  My kids love it.  We just share it  and use it for cheap plinkin, and teachin the little ones. Wish we had a second mag, but that is the only gripe with several hundred rounds of .22LR downrange.  

I think accuracy is the biggest thing.  I do not own a conversion or upper yet, but am looking into them.  the bore on a regular .223 barrel is slightly larger than what is optimal for the .22lr so you are not going to get the best accuracy.

Then there is the practice factor.  with a conversion you are just shooting your ar cheaply, but it is still your ar, vs a new upper that won't balance the same as your usual upper.