The Otter Creek Labs Polonium 5.56 suppressor is the full-size stainless steel Polonium, built for AR-15s and other rifles in 5.56 NATO and .223 Remington (Otter Creek Labs part no. POL556). It measures 5.8 inches long, one inch more than the compact Polonium K, and it wears a black finish. The rear of the can takes the universal 1.375×24 thread, which leaves the choice of mounting system to you. A 1/2×28 direct-thread adapter comes with it, matching the muzzle thread found on most 5.56 barrels. Stainless construction makes it a sensible pick for high-volume carbine work, where heat and wear add up quickly.

Key Specifications

• The caliber rating is 5.56 NATO and .223 Remington, the rifle cartridges this version of the Polonium is built for.
• Overall length is 5.8 inches, the full-size Polonium format.
• The body is stainless steel, a material that tolerates sustained rifle fire.
• The finish is black, a low-glare look that suits most AR builds.
• The rear thread is a universal 1.375×24 pattern, so you can pair the can with the mount system you prefer.
• A 1/2×28 direct-thread adapter is included, the standard thread on most 5.56 rifle barrels.

Frequently Asked Questions

How does the full-size Polonium compare with the Polonium K? The Polonium 5.56 is 5.8 inches long and the Polonium K is 4.8 inches. Both are stainless steel cans with the same 1.375×24 rear thread.

Can I direct-thread it onto my AR-15? Yes. It comes with a 1/2×28 direct-thread adapter, the thread pitch used on most 5.56 barrels.

Do I need ATF approval to buy this? No. As of October 9, 2026, ATF no longer requires Form 4 approval, registration or a tax stamp for suppressors. It transfers on a Form 4473 with a background check through a Class 3 (SOT) dealer. State and local laws still apply.

Buy the Otter Creek Labs Polonium 5.56 Stainless Suppressor, Black

As of October 9, 2026, this suppressor transfers on a Form 4473 with a background check at your Class 3 (SOT) dealer, with no Form 4 or tax stamp; state and local laws still apply. Order through Otter Creek Labs‘s suppressor lineup and our Lubbock, Texas team will handle the transfer, and it ships fast to your Class 3 dealer. The full-length tube is the one to choose when you want the most volume the Polonium line offers on a 5.56 rifle.