Upgrade your AR-15 carbine with factory-correct quality using the Colt AR6721 Receiver Extension – Part #160307105. This mil-spec carbine buffer tube is the same style of receiver extension Colt uses on its AR-15 and M4-pattern carbines, delivering proven reliability, proper fit, and long-term durability for serious use. Blackstone Shooting Sports+2Primary Tactical+2
Genuine Colt AR6721 Receiver Extension – 160307105
This part appears in catalogs and distributor listings simply as “AR6721 Receiver Extension” with Colt SKU / Brownells #160307105. It’s identified as a Colt-manufactured receiver extension designed for AR-15 carbines, not an aftermarket copy. Frisco Guns+2Brownells UK+2
Manufacturer: Colt
Product Name: AR6721 Receiver Extension
SKU / Part Number: 160307105
Style: Carbine, buffer tube / receiver extension Frisco Guns+1
Using OEM Colt components helps ensure correct geometry, thread spec, and extension length, especially important if you’re building or servicing rifles to Colt pattern.
Mil-Spec Carbine Buffer Tube – 7075-T6 Aluminum
Retailers describe the Colt AR6721 Receiver Extension as a mil-spec buffer tube built from 7075-T6 aluminum—the industry’s premium alloy for AR buffer tubes thanks to its high strength and excellent stress resistance. Blackstone Shooting Sports+1
Key construction details:
Mil-spec diameter means it’s compatible with the vast majority of mil-spec carbine stocks, giving you a wide range of buttstock options.
Collapsible Design – Carbine-Length AR Buffer System
Brownells and international Colt distributors specifically note that the AR6721 Receiver Extension is a collapsible-style, carbine receiver extension, intended for adjustable stocks rather than fixed rifle stocks. Brownells UK+2Brownells UK+2
Designed for carbine-length buffer system (carbine buffer & spring, not rifle-length components)
Works with collapsible carbine stocks that mount on a mil-spec diameter tube
Ideal for builds and upgrades where you want adjustable length of pull on AR-15 carbines
This makes it a great choice for modern AR-15 setups or for converting a fixed-stock carbine to a collapsible configuration, provided all other parts are correctly matched to a carbine system.
Compatible Colt & AR-15 Platforms
Product listings call out compatibility with a broad range of Colt AR-15 and M4-style carbines: Primary Tactical+2Brownells UK+2
Colt AR-15 Accurized HBAR
Colt AR-15 Carbine
Colt AR-15 Government Carbine
AR-15 / AR-15/M4 / M16 pattern lowers using a carbine receiver extension
In general, this receiver extension is built to fit standard AR-15 lowers threaded for the typical 1-3/16"-16 buffer extension threads, giving you Colt-spec alignment with properly matched components and stocks. Brownells UK+1
Ideal for Builds, Retrofits, and OEM-Correct Repairs
Whether you’re:
Completing an AR6721-style or Colt carbine build
Replacing a worn or budget buffer tube with a Colt-spec part
Standardizing a fleet of patrol rifles or training carbines
…the AR6721 Receiver Extension 160307105 provides a factory-grade foundation for your entire stock and buffer assembly. Its mil-spec dimensions support consistent buffer travel and alignment, which are important for reliable cycling and long-term wear characteristics.
What’s Included & Related Parts
Most distributor and retail listings treat this product as the receiver extension (buffer tube) only—supporting components are generally sold separately. Frisco Guns+1
Common related parts (often listed alongside this tube):
Receiver extension / castle nut (e.g., AR6721 Receiver Extension Nut 160307103) Frisco Guns+1
Receiver end plate (AR6721 Receiver End Plate 160307102) Frisco Guns
Carbine buffer & spring matched to your gas system and barrel length
Mil-spec collapsible stock of your choice
Always confirm exactly what’s in the box with your retailer so you can order any needed hardware at the same time.
Technical Specs – AR6721 Receiver Extension 160307105
If you’re looking for a true Colt-spec mil-spec carbine buffer tube to anchor your collapsible stock and buffer assembly, the Colt AR6721 Receiver Extension 160307105 is a proven, OEM-correct solution for AR-15 carbines.
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