Mil-Spec dimensioned The RE-10/A5SR receiver extension tube is designed to work with the Vltor A5 Rifle Recoil System, and can also be used as a replacement receiver extension tube for the AR-10 Rifle. It is important that you understand the requirements for the additional components, based on the intended application. Physically, the A5SR tube is slightly longer than a traditional AR-15 receiver extension tube and is functionally the same length as the AR-10 receiver extension tube. The A5SR tube has six locking positions and is designed to work with a MilSpec, collapsible stock, such as the Vltor Improved Modstock. If you are using the A5SR tube as part of the Vltor A5 Recoil System on an AR-15 type rifle or carbine, it must be used in conjunction with a Vltor A5 Buffer & Spring. These parts are NOT the same as standard carbine or rifle receiver extensions or buffers. Attempting to use these parts with standard carbine or rifle recoil systems will result in damage to the firearm! If you have any questions about these parts, please contact Vltor Weapon Systems at 866-468-5867 before installation. The standard A5 Buffer and Spring Kit, as weel as special purpose heavy A5 buffers (A5H3 and A5H4) can be purchased directly from the Vltor webstore. NOTE: While the A5SR tube will work on the AR-10 Rifle, the two are not identical. You should NOT try to use an AR-10 tube as part of an A5 Recoil System. If you are using the A5SR tube on an Armalite AR-10 carbine and rifle, you must use the Armalite AR-10B Operation Spring (Armalite Part #EA1095) and AR10 Carbine Buffer (Armalite Part #ECO120). Attempting to use this tube with parts other than these, will result in damage to your rifle! If you have any questions about these parts, please contact Vltor Weapon Systems at 866-468-5867 before installation.