Thanks to a recent post by JayG at Stuck in Massachusetts, a discussion was started referring to the Hipoint 9mm carbines that have been on the market for a few years. I have one of these carbines, and I absolutely love it. It's cheap, accurate, reliable, and with the ATI stock on it, it looks pretty decent.
The only issue I have with it is the low magazine capacity. The factory mags are limited to 10 rounds, and the aftermarket 15 round mags I have had all sucked. An idea that has run around the internet gun boards before was to modify a Suomi M31 72 round drum to work with the Hipoint carbine. The problems I've read about is that the feed lips are too weak to support the wound spring pressure from the drum, and the rounds often don't point at a sufficient angle to feed reliably. I've often thought about building one, but just haven't done it. I figure now is a good enough time to start, since I only have a small bit of tinkering to do with the .50 anymore.
I will not bother with a factory Hi point magazine. They are very thin sheetmetal and just aren't strong enough to take repeated positioning and welding, not to mention the increased spring pressure from the drum.
To start, I plan to use some 4130 sheet steel to build a custom magazine so the feed lips can be hardened. Unfortunately, I don't have any 4130 sheet laying around, so I am going to make my prototype out of standard cold roll sheet I do have laying around.
The next thing I must do is grind off the feed lips and remove any burrs and rough spots inside the drum that would keep it from feeding smoothly, and align the new mag with the drum so that there is a smooth transition from drum to magazine. The mag must be positioned such that the bullets point up at a certain angle so that they will feed into the chamber appropriately as well. This may require a feed ramp inside the rifle, not sure yet.
Another consideration is that the total weight of the loaded drum is more than the factory mag catch can handle, and I bet that the area surrounding the mag catch is not capable of that much stress, either. So, I will have to redesign a way to secure the drum to the rifle. This will likely be the hardest part of the conversion from my standpoint, because whatever is built needs to integrate well into the overall design of the rifle. I am at a loss on how I will do that, but it may come to me over time.
Keep an eye on this little corner of the internet if this is something that appeals to you.