Forward Controls Design A5 Castle Nut, Black

The Forward Controls Design A5 Castle Nut in black is a precision-engineered receiver extension fastener purpose-built for the Vltor A5 stock system. If you're running an A5 setup and want a castle nut that actually stays put and won't give you grief during installation, this is the upgrade over standard TDP spec nuts. FCD engineered the CN5F specifically to let even longer carbine stocks fully collapse on Vltor's RE-A5 and FCD's RE5F receiver extensions, giving you the adjustment range your rifle deserves without compromise.

What makes this castle nut different starts with the details. The staking notches—the part you actually use to lock the nut in place—are 25% longer and deeper than standard, cut at a shallower 30-degree angle instead of 45 degrees, and there are four of them instead of three. That means you've got options when it's time to stake, and once you drive those pins home, this nut isn't going anywhere without deliberate action. The coarse serrations on the wrench side aren't just for show; they give you actual grip when you're torquing it down toward the end plate, so your fingers don't get shredded on sharp wrench notches. FCD believes some things on an AR should stay locked down, and the CN5F is engineered that philosophy.

Machined from 4140 PH steel and finished in black nitriding, it's built to the same exacting standard as every FCD product and manufactured right here in the USA. At 0.51 ounces, weight isn't a concern, but durability absolutely is.

Key Features

• Extended length (0.1" longer than standard) designed for full stock collapse on A5 receiver extensions
• Four staking notches cut deeper and longer (25% longer than TDP spec) with 30-degree angle for more secure locking
• Coarse serrations on wrench side for better grip during installation
• Billet machined from 4140 PH steel for durability and precision
• Black nitrided finish, made in the USA

Specifications

Frequently Asked Questions

Is this castle nut compatible with Vltor A5 stocks?

Yes. The CN5F is designed specifically for the Vltor A5 system and FCD's RE5F receiver extension. It allows even longer carbine stocks to fully collapse on RE-A5 and RE5F extensions. While it will fit standard TDP spec length receiver extensions, the additional length may prevent full stock collapse on those shorter tubes.

Will this work on standard AR-15 receiver extensions?

The CN5F will physically fit standard TDP spec receiver extensions, but the extended length means your carbine stock won't fully collapse as designed. It's purpose-built for A5 systems; use standard length castle nuts for milspec receiver extensions.

How many places should I stake this nut?

Forward Controls Design recommends staking in two places, per technical manual. The CN5F has four staking notches, which means three will always be visible and available for staking, giving you flexibility in where you place those two stake pins.

What's the difference between this and a standard castle nut?

The CN5F extends the TDP spec design with longer and deeper staking notches (25% longer, cut at 30 degrees instead of 45), four notches instead of three for more staking location options, and coarse serrations on the wrench side for easier installation. It's optimized to work reliably with A5 stocks and receiver extensions.