Times like this I wish I was not so paranoid in my younger days. Yes, it can be done, I have done it.
Look at an AR Lower, imagine cutting it into three sections:
Trigger Group section, Magwell group Section, and Center support section.
Trigger Group Section: Obviously, this contains the FCG, provides a place at the rear to attach a sort of rear trunnion, which the buffer tube attaches to, and in the front, it attaches to the Center support section.
Center Support Section: This is the tricky one, it contains the mag release, and the Bolt Hold Open. The Trigger group section mates directly to the Center Support Section, and the Mag Well section lays over the top of the Trigger Group Section.
Mag Well Section: Obviously, This is the section that both holds the magazine, and the front pivot pin section. The rear of this section should overlay the front of the Trigger Group Section.
Once the needed machining is done to the Center Section, the three sections can be joined either by spot welding, or by drilling and riveting.
The trigger group section should have a slight rail fold on the top, just like an AK for stiffness.
The Rear pivot pin goes through the rear trunnion, which has a channel drilled from the top, then milled out to allow the upper receiver rear hole to rest inside. Ends up working just like a normal AR.
The front Pivot is milled out from bar stock, or bent and folded depending on how you want to retain the pin. If you are OK using c-clips to retain it, then just bend it out of sheet steel.
That is about it. I made two of these back in the late '90s. I had intended to make another that was modified to accept AK-74 mags, but just never got around to it.
I was surprised at just how forgiving it is to work with steel in this way. I used an Olympic Arms lower as a guide, measured a hundred times, and then cut.
Now, I did this the hard way. I wanted to make an AR class lower, that would cosmetically be equivalent to an standard AR. The lower ended up weighing almost 2 lbs. OUCH!
Take a lesson from the AR-180 design: Keep it simple, and do not care about the cosmetics.
Here is a link to pics that will help illustrate what I am talking about:
AR-180 - 5.56x45
You can all see that IF we were to create a lower receiver template, and SOMEONE created the stamping dies, it would be fairly simple to press these out, dimple them for the holes that need to be drilled, and sell them for a decent profit. Even if it was just a straight walled lower, it would not be difficult to produce the spacers and longer pins for the hammer, and trigger. The hardest part would be the selector, but it could be solved by adding a right side selector lever and screwing them together.....Just a thought.
Regards,