I spent a couple hours on the single stack magazine and Suomi drum configuration tonight. So far, it seems to work just fine. I say so far, because I didn't have enough 9mm in my garage to fill the drum up to max capacity, and all my ammo is in my bedroom in .50 cal ammo cans a few feet away from my sleeping spouse, so I just used the ammo from one of my carry guns and the ammo I had in a factory Hi point mag, so 27 rounds total.
The only issue I had was that the bolt face and feed ramp are both VERY dirty, and need cleaning and polishing. These two in conjunction caused the bolt to load the fresh round, but slowly, as the round could not eject from the magazine and move up the feed ramp smoothly. Some CLP and a scotchbrite pad on my dremel will take care of this.
With that said, if I cycled the bolt quickly, I got every round to chamber just fine. No jams.
To construct this so far, I cut up a 15 round Promag magazine I had laying around that didn't work, (seriously, the only thing worse than the factory crappy magazines is the more-expensive-jam-o-matic 15 round magazines) and then cut the top off of a factory 10 round mag to weld in place on the top of the Promag. Before I could do this, though, I had to get the angle of the mag in relation to the drum correct, so the rounds would point up enough to be stripped off by the bolt, and so they would feed smoothly from the drum into the magazine. It took a couple tries to get the mag angle right, but it seems to work well where it is.
In this pic, you can see how the drum and magazine are offset. Instead of centering the mag over the drum, I offset it so that there was not a sharp bend in the drum-to-mag transition area. It looks a little goofy, but I really don't care, so long as it works.
Looking down from under the barrel at the drum locked in place on the carbine.
This round is just slightly pointing down, but not enough to keep it from being stripped from the mag and chambered.
There is a pretty drastic angle between the drum and magazine, but that is the determining factor in getting the rounds to load right into the magazine.
So, it's all tacked together, and I may take it to the range this week and test it after I clean the inside of the gun real good.
For those of you wondering if the feed lips are strong enough to hold up against the drum spring pressure, they are not. However, if you do a little welding and reinforcement on them, they are stiff as heck and can't be bent, even with a pair of pliers.
Even though I think it's going to work just fine, I'm not real happy with it. There is a lot of work involved in doing this correctly, and that involves hacking up a factory mag and an aftermarket mag just to get to an end product. While I probably could have just made a new mag on my metal brake, I really didn't feel like it. I did it this way to see how feasible it would be, and if a single stack arrangement could be made to work, which I think it can. The thing is, I think half the headaches I had while throwing this setup together were a result of trying to make it all work with a single stack magazine.
I will probably try to test fire it tomorrow or perhaps Wednesday, if i can. If all goes well, I may leave well enough alone, and try my hand at building another. However, if I still don't feel 100% about it, or if I find a good reason not to make another, I am just going to go ahead and modify my rifle to use double stack magazines, and try to build a drum assembly from there.