Dead Foot Arms MCS 4" AR-Pistol Kit for rifle calibers is our short tube conversion kit that features our patented, dual spring, bufferless technology and uses your bolt carrier group! It includes: Buffer Tube and End Cap Assembly, end plate with QD port, plunger, and recoil and buffer spring. If you are going to use your own BCG with 9mm, and keep it super compact, this product is the shortest receiver extension you can run, period. No one else is even close. This is exactly what you need. Note- This product is compatible with the Dead Foot Arms SCW 4 Tailhook and Stock to easily make a compact PDW upgrade. Compatible with most 9mm AR-15 pistols and rifles: BCG needs to be the full-length style and have a removable internal weight to work with the MCS. For CMMG Radial Delay you will need to order the Rifle Caliber version.