I know there are some downsides to a 9mm blowback AR.

I know there are some challenges to a 9mm DI AR.

Why not pursue other ideas such as other forms of gas operation or various forms of delayed blowback or extended chamber or advanced ignition?

Also, what if reloading the brass was not an issue and the brass could be considered sacrificial?

For example, there have been firearms with rings in the chamber and others with spiral grooves in the chamber.

TED