An interesting question was brought up by Rick at http://ar180s.com/ a while back about making new AR-180B upper receivers. The actual receiver is not much of a problem however the trunnion is. The expense of making a new trunnion would be cost prohibitive so a different solution had to be found. In this case the thought would be to do what was done, or thought to be done by Armalite and that is using an AR-15 barrel extension pressed into a new trunnion block. So looking at a AR-180B we can see what the trunnion looks like in a receiver.
There are a number of differences. The original is forged and then machined. You can see how the boss sticks out in front of the bolt locking area. While the 180B the rear of the trunnion is flush.
The reasons for these differences is because a AR-180b does use a AR-15 barrel extension pressed in. However there are changes to the barrel extension.
In this hidden view you can see that a AR-15 barrel extension would be to large for a trunnion as originally designed for an AR-180
This can only work if the trunnion is also re-design. This is what Armalite did for the AR-180B.
As you can see in this cut away the reduced size of the outer ring of the barrel extension.
In this picture you can see the hole for the guide pin on an AR-15 barrel extension.