The Delta Series AR-15 Upper Vise Block is optimized for left hand and right hand Mil Spec A2 upper receivers with carry handles, as well as flattop style receivers. Other AR-15 variants may be adapted with special attention to potentially non-conforming surfaces or features. The Block allows you to safely clamp on to your AR-15 upper receiver in a vise holding it securely without twisting, crushing or marring the finish. This Block is the perfect product for use in torquing a barrel, removing a stubborn barrel nut, or for simple gun maintenance and cleaning. It is made of a high pressure, solvent resistant polymer material which can withstand almost anything you can throw at it. The Delta Series AR-15 Upper Vise Block truly is the all-around product for your AR-15 gunsmithing, maintenance, or cleaning needs. As an added bonus it includes a receiver insert with a gas tube alignment system, making sure your gas tube is precisely positioned when installing a barrel and torquing a barrel nut. Durable design holds receiver securely. Receiver insert with gas tube aligner included.