The shortest uppers we make: a 4-inch Stern barrel under a 4-inch MOD4 M-LOK handguard, in 9mm, .40 S&W and .45 ACP. Each upper is assembled and test fired before it ships, and it pins onto the mil-spec AR-15 lower you already own.
Pair the upper with the magazine adapter for the magazines you want to run: the MAG-AD9 for Glock 9mm and .40, the MAG-ADMP9&40/P320 for S&W M&P 9 and 40 or SIG P320, and the MAG-ADMP45 for M&P 45. The pistol upper kits on this page include the upper and the adapter together.
Short barrels are hard on buffers. Run an H3 buffer, 5.4 ounces or heavier, for 9mm, at least 7.5 ounces for .40, and 11 ounces for .45. The 4-inch .45 in particular should not be run on a standard carbine buffer.
Check your state and local laws before you build. Our state compliance page collects the resources we point people to, and our compatibility page covers lowers, bolts and magazines.