It's the buffer tube that a rifle stock or pistol brace attaches to. This would be classified as an SBR if it had a butt stock so instead it's been built as a 9mm pistol carbine. I now have a pistol brace installed on it but it was... nekkid 😝 for this photo.

It's the buffer tube that a rifle stock or pistol brace attaches to. This would be classified as an SBR if it had a butt stock so instead it's been built as a 9mm pistol carbine. I now have a pistol brace installed on it but it was nekkid for this photo.