Starting to put things together and determine overall length. Here is the bolt in the upper:
The bolt face is about half an inch into the receiver, when the bolt is locked, so I need a 17" barrel beyond the end of the upper to be legal.
If that looks long its because it is. Here is the same setup with my AR15 for scale, and to show how much the stock will add.
Crappy picture, but its clearly a long rifle, the AR15 has a 16" barrel, and is 32" OAL. Because the AR45 is based on an AR-10 receiver its two inches longer.
This leads to the obvious conclusion that this rifle will have to be a SBR, otherwise it will be too damned big to be at all 'handy'. That still leaves the question of suppressing it, but having a suppressor, and an SBR may not add much, other then a bigger check to the BATFE.
Finally here is the receiver with a mag installed. That's a converted AR-10 mag that will feed .45ACP.
The next step is a lower parts kit, a stock and a charging handle.