Here is a 15-22 built specially for speed steel shooting. It shot the Ruger Rimfire World Championship from 2011 to 2013, and the NSSF Rimfire World Championship in 2014 plus numerous other steel events that allow .22 rifles, including Steel Challenge.
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The most important thing you need to change is the trigger and its pull weight. Minimum is to install a set of the JP "yellow" low power trigger and hammer springs ($10). That will reduce your trigger pull from as much as 5-6 lbs. to as low as 3 lbs. Maximum is to install an AR trigger designed for 3-gun competition, the one most use is the Geissele Super 3-Gun (around $250). This will allow you to pull the trigger fast enough to be able to empty a 25 rd mag in about 4 seconds. But that is not why you use it. It has a very fast lockup and a short reset which means you don't have to release the trigger all the way to fire the next shot, just relax your trigger finger about 1/8" and you can squeeze another shot off.
The most reliable ammo for a rifle shooting speed events is CCI AR-Tactical. It cycles better in inline magazines because it has a narrower tip (it actually uses the bullet from CCI Standard Velocity match ammo with a copper wash and more powder). It also feeds 100% in Ruger 10/22 rotary magazines.