Great feature of the AR15 platform is that you can make many rifles from one receiver. For those shooting High Power matches with their Service or Match version of the 15, a 22LR upper is a real aide in practice shooting to develop all those skills needed. Various mfg's out there but would suggest you make sure the barrel is the right twist, not the std. 7,8,9, etc. Believe the 22LR is normally twisted at 16 or so. Using 22 ammo is just fine at 100 yards and certainly helps or forces you to watch the wind and conditions. Off season or during the season and by using the 22LR the costs for shooting is a heck of a lot less. I use a Compass Lake version and can say that the accuracy is right there on a level with my 40X small bore, so you won't lose any performance. Just a suggestion.

I use a M-261 conversion kit in a 1-12 pencil barrel chrome lined colt A1 upper.. works great on ground squirrels.. will group under an inch off bags at 50 yds.. scope on carry handle lets me see what I'm shooting at.. cheap to feed ... love it!.. Les