Lapping an AR15 Upper Receiver

This is our stripped YH upper and a lapping bar designed for AR15 type rifles.
Lapping an AR15 upper receiver ensures that the barrel is true to the upper receiver by providing a perfectly perpendicular surface for the barrel extension.  This task is easily accomplished with a few simple tools and a handheld drill.
For this project, Brownells supplied us with the following:
This is our stripped YHM upper and the AR15 lapping tool.
The lapping bar requires silicon carbide abrasive. 180 grit is recommended.
The upper is secured in a set of blocks from our Critical Tools Kit. The portion that contacts the inside of the upper is coated in oil. Once inserted, the 180 grit abrasive is placed on the contact surface of the lap. NOTE: It is critical that the abrasive does not enter the inside of the receiver.
We use a power drill to turn the lap and contact the face for a few seconds. We look at the face to assess progress and then contact again as needed. Ideally, you want at least 80% of the surface lapped.
Here is the finished surface. The front of the receiver is now completely true.