This is the Smith & Wesson 10 round full size .22LR magazine for the S&W M&P15-22.
The 10 round version of the 25 round magazine it is internally restricted to 10 rounds for ranges and locations with capacity restrictions.
Using M&P 15-22 magazines in other AR15 lowers?
M&P 15-22 magazines are popular in AR15 .22 rifles fitted with last round bolt release catch mechanisms, including the KRISS Defiance DMK-22 which comes with one as standard.
Please note M&P 15-22 magazines are slightly shorter front to back that AR magazines (by just over 2mm) so work best with special shims fitted.
See our related accessories:
Catch22 AR15 .22 bolt release catch upgrade
Better-Mag S&W .22 Rimfire Magazine Adaptor for AR15 .22