I was searching for a 357SIG carbine / a way to turn a pistol into PDW to go with my M357 A1 for the benefit of having a pistol and a rifle in the same caliber. It would be nice if the magazines were interchangeable but that's less important for me when the presence alone of any guns in 357SIG is appreciated. Searching in general for any semi-auto chambered in 357SIG with barrel longer than typical pistol length, I realized Glocks and Sigs had strong aftermarket support although they don't make it from the factory. Some 6" barrels were in stock but I believe I could order them longer and threaded. Then I start looking for PDW platform. Ruling out the Roni kit since I didn't like how the gun is braced to the body, I came across a product called X01 by Fire Control Unit for P320. It uses everything from the "donor" pistol down to the magazine release except for the polymer grip frame. X01 link : https://firecontrolunit.com/?product=exo-one-320-grip-module If you have field-stripped your Steyr and went one step further to remove the trigger group from the steel frame(don't lose the slide lock spring lol), this will make a lot of sense to you. Since P320's trigger group is what is considered "the gun", we can swap out the polymer grip frame with something that is easier to hold and aim with. Just like you can put different grips and stocks on your AR15, which is the receiver. I didn't care too much for IGB Austria's Steyr shoulder brace, nor the Roni kit but this PDW platform taking advantage of modular construction of a pistol really makes me drool. Not really sure on the legalities of fitting the X01 to work with Steyr since we have serial numbers on more than just the trigger group, but I would like to take on the project if I was able. Any chance Steyr starts marketing "Hey we have modular lego pistols too! I mean rifles! I mean both! This is both!" with more barrel options..?