Receiver Extension

A receiver extension, commonly called a buffer tube, is the cylindrical aluminum component that threads into the rear of your AR-15’s lower receiver. This critical part serves two primary functions: housing the buffer and buffer spring assembly that cycles your rifle, and providing the mounting point for your buttstock.

Understanding Receiver Extension Types

The two main categories you’ll encounter are mil-spec and commercial receiver extensions. Mil-spec tubes measure 1.14″ in diameter and feature a harder anodize finish, while commercial tubes measure 1.17″ in diameter. This distinction matters because your chosen stock must match your tube diameter; they’re not interchangeable.

For AR pistols, specialized pistol receiver extensions are shorter and don’t include the adjustment positions found on carbine tubes. A standard carbine receiver extension features six position slots, allowing approximately 3.25″ of length adjustment for your buttstock.

Why Your Choice Matters

Your receiver extension directly impacts several aspects of your build:

• Buffer system compatibility: Carbine extensions typically require carbine-length buffers (2.9″), while rifle-length tubes need rifle buffers (5.2″)
• Stock selection: Your tube diameter determines compatible buttstocks
• Overall length: Different extension lengths affect your rifle’s collapsed and extended dimensions

Installation Tips

When installing your receiver extension, proper torque is essential. Most manufacturers recommend 35-40 ft-lbs for the castle nut. Always use proper staking to prevent the castle nut from loosening during use. The endplate should sit flush against the lower receiver, with no gaps visible.

When using the AR15 Outfitters builder, filter by your preferred tube type early in the process. This ensures all subsequent component selections, particularly stocks and buffer systems, remain compatible with your chosen receiver extension.