Okay, we are starting here with the ATI stock. Looking down from the top and up from the bottom of the grip, we can see that there is a LOT of room for a double stack magazine. The slot for the single stack magazine is actually a bit larger than the magazine needs to be, so from this perspective, the conversion to a double stack magazine is very much a reality. You can't easily see from these pics, but there is quite a bit of room on both sides of the receiver between the receiver and stock. This may come in hand later if/when it comes time to widen the receiver to accept a double stack magazine.
Now, here is the factory single stack magazine in place in the stripped receiver. This is how high the double stack mag will have to sit as well. This will pose a distinct problem due to the width of a DSM.
Here is the top of the factory Hi Point mag in relation to the feed ramp. Nothing special, but we're starting to see that a DSM fitting in the factory slot here just isn't going to be a quick mission with a dremel tool.
The XD mag for comparison of width. This is an XD40 mag, but they are the same width as a XD 9 millimeter. I know this because I shoot 9mm, 40 S&W, and .357 sig out of my XD, with different barrels of course. Anyway, again we see that the width of the mag will exceed the width of the receiver.
Here is the simplistic trigger assembly. There just isn't a whole lot here, folks. That transfer bar is going to be the biggest issue in widening the receiver. No big deal, it just means I will need to make a new one or modify the existing one.
Now, what I am proposing might seem a bit odd or even stupid, but I have reviewed the shape and material of the receiver, as well as where most of the stresses are on the receiver. This receiver material is cast Zamak, which is pretty much just pot metal. It is die cast, similar to construction techniques used in making hot wheels cars and other cheap metal objects. My hypothesis is simple; I could easily build a widened receiver section out of 1/8" thick steel that would bolt to the carbine receiver and then be silver soldered in place. Given that it was securely fastened and soldered, I imagine it would be fairly strong. Most or all of the stresses present in this carbine are all forward of the area where the magazine is retained. The only real force seen behind the mag area is the spring tension present when the striker spring is compressed and the sear is holding the striker in place. I am not fond of the idea of tinkering with pot metal, but the good thing is that it would be very difficult to modify this rifle and make that area weaker. There just isn't that much factory material in that area. In fact, I'd be willing to bet that my idea would make the rifle much stronger compared to how it is now.
After the receiver was widened and the offending area preventing insertion of a DSM was machined out, all I would need to do is make a new transfer bar for the trigger and do a little modification of the mag release system. That wouldn't be too hard.
Now I just need to chew on it and think if I want a dual stack magazine or not. I imagine that my choice whether or not to pursue the DSM will hinge on how well a Suomi drum/single stack mag works out.
Now, the Suomi drum...
Here's the inside of the drum with the spring at rest and the follower all the way up to the feed lips.
The drum fully wound. One this thing is fully wound, you just dump your 9 sillimeter rounds in, facing up from this perspective.
Now, were I to pursue a setup that utilizes a magazine that is actually part of the carbine and has a drum mount on the bottom, the feed lips each drum would be filed off. To load the rifle, you would fill the drum to max capacity,fill the inner magazine to capacity, put the front plate on the drum, mount the drum on the carbine, and then push the button on the drum to put tension on the rounds. Lock the drum face plate and blast away. I like the idea of building it this way solely for the purpose of making it easier on people to have multiple drums, loaded and ready to go.
Going the other direction and welding a magazine tower directly to the drum would be pretty easy, but limits the shooter to having one drum, unless they wanted to buy more than one modified drum. It also means that if I built these, I'd have to keep a decent supply of drums on hand.
I'll probably get started on the welded drum/single stack in a few hours, and hopefully I'll have a working prototype before too long. If I can't get it to work without having to completely reinvent the wheel, I may as well modify the carbine to accept a DSM.