For general practice out to 100 yards a Ciener Conversion unit will genraly work very well in most AR-15's. If you have an M-16, he makes a full auto model. In any case I'd get the 30 round magazine unit as it is a lot of fun. A friend bought a "dedicated upper" for considerably more money and it was somewhat of a disapointment, as it did not work much better than the the Ciener "insert type" unit. As a side note... most 22's do prefer certain types of ammo. In my Ciener I have the best accuracy, (plus or minus 2" at 50 yds,) with winchester wildcats. The more expensive, plated super x 22's also work very well. Remington thunderbolts, in several of my guns, are terrible. They fouled up a ruger 22-45 so bad I though it ruined the barrel.