The B-9 does not come with a built-in picatinny rail. It comes with a specialized rail for the RK-series rail-mounted vertical grips that allows them to be mounted directly to the B-9.

Rail-mounted extender allowing for flashlights to be mounted further front of the handguard.

UPDATE: B-9M available - slightly more compact with same placement, differs from standard B-9 in weight and lack of additional accessory mounting capability (no grip interface like on standard B-9).

B-9 / B-9M Goose Light Mount