What are the internal receiver rails that the bolt carrier group rides on made of? Are they steel, aluminum, plastic?

Plastic like the rest of the upper.

Beretta went with steel or plastic for pretty much everything. The only aluminum is in the rails, selector switches, and the barrel support up front.

AJ

Thanks for the info. I haven't found anything about that be a problem so I assume it is good to go. Just asking because I have been thinking about getting one.

I've got about 600 rounds through mine. The bolt carrier paint has polished up where it runs on the receiver while the receiver has no significant wear.

The highest wear area is the bolt extension steel where the parkerizing had worn through with some metal wear.

Some pics showing wear here: http://www.bobistheoilguy.com/forums/ubbthreads.php/topics/3688167/Beretta_ARX_100_design_details#Post3688167

AJ