Wrenches for installing AR-15 barrel extensions are extremely specialized, and ridiculously expensive. The wrench described here is suitable for torquing the barrel extension to the 140 ft/lb specification, and will also work for grabbing 1″ barrels as well.
I started by cutting out a couple of pieces of 3/4″ thick material. The dimensions aren’t critical as long as there’s room for the holes – my wrench is about 4″ long, and that’s a good dimension. The blocks should also be at least 3/4″ wide. Square the two up so they are uniform in size and have smooth, flat surfaces.
1/2″ in from each end drill #7 holes. Repeat for the second piece. On one piece, open the holes to 1/4″ and counterbore for socket-head cap screws. On the other piece, tap the holes 1/4″-20.
You should now be able to connect the two halves with some 1/4″-20 Socket Head Cap Screws (SHCS).
Now layout positions for two holes – a square hole for the torque wrench, and a round hole for the barrel or barrel extension. Keep at least 3/4″ away from the screw holes.
Now to make the square hole. Simply take a wrench half, and lock it in the mill vise while holding it at a 45° angle.
Now eyeball the bottom of an endmill even with the line scribed on the block and start cutting. For a 1/2″ drive wrench you want to go a little over 1/2″ in the X direction.
Properly done, the wrench halves should form a square hole when complete.
The next step is to bore a 1″ hole. Bolt the two halves together and bore both sides at one time. This can be done on the mill, with a 1″ endmill or a boring head, or done in the lathe if you have a 4-jaw chuck.
With the hole bored, the wrench is complete.
To use, I place some paper shims made from index cards in the 1″ hole. I cover these with rosin, moisten the rosin, and clamp the assembly to the barrel or barrel extension. Between the stiction of the rosin and the clamping force of the screws, this wrench really holds.