The Smith & Wesson M&P 15-22 is an excellent .22LR training rifle in part because it has great magazines. The mags are inexpensive, reliable, easy to load, and have a bolt hold open feature. Redi-Mag seems to like the M&P 15-22 magazines as well because they created the Better-Mag S&W .22 Rimfire Magazine Adapter.
The Better-Mag S&W .22 Rimfire Magazine Adapter is a simple insert that can be secured in the magazine well to allow you utilize M&P 15-22 magazines in your .22LR AR-15 trainer. It fits any trainer that uses a standard AR-15 lower. So, if you use a dedicated .22LR upper or a conversion kit, this should work for you.
Check out the Better-Mag S&W .22 Rimfire Magazine Adapter at Redi-Mag.com.