A .40 S&W upper from Stern Defense pins onto a mil-spec AR-15 lower and runs the .40 magazines you already own. The MAG-AD9 adapter handles Glock .40 S&W magazines; the MAG-ADMP9&40/P320 handles S&W M&P 40.
The .40 line runs in three lengths: a 4-inch M-LOK pistol upper, an 8.5-inch M-LOK upper, and a 16.1-inch carbine. Each is assembled and test fired before it ships, with a Stern Defense .40 barrel, a bottom-feed bolt, upper receiver, handguard, muzzle device and charging handle.
Buffer weight for .40 sits between our 9mm and .45 recommendations: run at least 7.5 ounces, and go heavier if you shoot suppressed. We do not make a 7.5-ounce buffer; it is a standard weight carried by most AR parts retailers. The adapter and upper are also available together as a .40 S&W conversion kit, and .40 barrels are sold on their own. Compatibility notes are here.