Just to keep all the receiver extension info in one searchable place, in addition to the one above, these are also known, proven, receiver extensions from reputable manufacturers. Internal depth of these extensions is 7 5/8".
1. Armalite AR-10 Carbine Receiver extension, Armalite Part number: 10207025
(The entire receiver extension kit from Armalite is Part number: AR10REKIT01,and it's much cheaper than sourcing the parts individually)
2. VLTOR Part number: RE-PA5, which is a pistol receiver extension - would be used on the short-barreled Big AR Pistol you intend to build...
3. VLTOR Part number RE-10/A5SR, which is a 6-position Carbine receiver extension.
4. VLTOR Part number RE-A5, which is a 7-position Carbine receiver extension.
5. Magpul® Enhanced Receiver Extension – SR25/M110, which is a 10-position Carbine receiver extension.
6. BCM Intermediate Receiver Extension (A5 Buffer Tube).
Everything in this thread right now, to this point (with Rob's extension in the first post) are all verified at 7 5/8" internal depth, and all will operate in place of the Armalite AR-10 Carbine Receiver Extension on a .308AR, or interchange into the VLTOR A5 System for small-frame ARs (this is almost all I run on the Grendels).