Wednesday, July 13, 2011

A quick (I hope!) project.

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.


  1. I had a thought about the weight issue. The rear receiver cover screw is the nearest and most convenient load bearing thing I could think of. A somewhat u shaped piece that fits underneath there, supporting a piece contoured to the back of the pistol grip that ends at the base of the mag well with a second load bearing mag catch setup. One that mates with the appropriate Macgyvered second catch setup on the mag. I hope that rambling description of my idea was clear enough.
    A replacement rear bolt might be necessary to allow for appropriate thickness of the load bearing piece at the connecting point.
    As for the possible feed ramp issue, I may be stuck on stupid but I had a thought that maybe cartridge angle issues could possibly be addressed with the design of the follower.
    Anyhow, I'm not a gunsmith and don't play one on tv so I just figure to throw some stuff out there.

  2. Well, after some brainstorming this evening, I have a new idea for the whole assembly. I have heard too many horror stories of people ordering the Suomi drums and receiving one with worn out springs, beat up casings, and a whole host of other issues. If I were to seriously consider manufacturing these, I wouldn't be able to absorb the time and trouble of hit-or-miss quality on the drums.
    So, why not set it up so that the drum itself is not heavily modified, but is able to attach to the carbine in such a manner that I don't actually have to mess with the drums at all?
    The way I've figgered out so far is to make a "magazine" that is integral to the rifle itself, that literally drops in from the top of an disassembled rifle, bolts in place, and has a Suomi drum mount that bolts to the bottom. This "magazine" would be machined out of steel and between bolting to the inside of the receiver, I could add an external mount to assist in spreading the stress of the loaded drum's weight.

  3. I'm not opposed to the integral mag idea, in spite of the inability to use standard mags. One question I have is how would the standard Suomi drum feed up in to that mag set up?

  4. Another thought I had on the weight bearing issue is the metal support inside near the bottom of the mag well. If you go with a detachable version of the mag, perhaps a replacement part for that piece could be fashioned that includes a second European style mag catch at the bottom of the mag well.

  5. Dave_H,

    If you had an integral magazine, you could merely grind the feed lips off of the drum. Remember the Suomi drum spring has a release, and there is no spring pressure on the rounds until the release is pushed in. This would push the rounds up into the magwell of the carbine. You could load 10 rounds (or however many you needed to fill the carbine handle) before attaching the drum. After tipping the rifle up and loading it, attach your drum and hit the drum spring release. Tada, ~72 rounds of 9mm.

    As far as a euro-style mag release, that is a great idea, but unfortunately the inside of the carbine handle is all plastic, so it probably wouldn't be worth the extra work.

  6. Ok. Well I don't own one of those semi-auto Suomi carbines, so my knowledge of the drum mag is limited.
    As for the latest part I suggested, if the part had a piece that went up and attached to the frame in some way I think it would work. It seems only a slight amount of plastic removal inside the grip would be necessary to make room for this hypothetical part.
    Anyway, I'm just throwing some ideas out there.

  7. Okay, I'll try and post a picture or three of the drum later so you can see what I am talking about.

    Attaching to the frame is a good idea, and I like your thinking. The problem is, the part of the frame directly below the moving parts of the receiver is made of polymer plastic (I think that's polymer plastic, it's kinda hard to tell given that it's a Hi Point! :))
    So, the only way I can see it being secure would be to mill out the grip area a bit and make a sleeve that fits in there and is mounted in several places to the grip. That would spread the weight of a euro-style mag catch out over the entire grip, and would probably work. However, that seems like a lot of work (not that I'm against it, I'm always up for new ideas) for what we're trying to do here. However, I will certainly do a little brainstorming on the matter and see if I can polish the idea a little. It is a good idea, to be sure, just not real sure how well I can apply it.

    I guess it goes without saying that this wouldn't even be a remote consideration with the factory stock, it is paper thin and flimsy as hell. I refer to the ATI stock as the basis for all the modifications. It is much thicker and of better quality.

  8. I didn't have one of my carbines apart when I was thinking about it. I'll take a look, fwiw. Maybe a single piece of steel running up to a connecting point on the frame from the mag catch assembly would be simple way to go.
    Assuming there is a good place to attach it up top. Of course even if that works, I guess there would have to be a way to easily disconnect it from the mag catch assembly to allow for normal takedown of the carbine for cleaning.

  9. LOL, well, about the most I've done so far in the last few days was to sit and stare at the disassembled carbine and daydream.
    Although I'm sure I can get something working along the lines of a single stack mag, I'd be willing to bet that if I actually do go the dual stack route, I would build a beefier mag catch that would alleviate most or all of the problem.

  10. Let me tickle this thread..

    This subject has once again surfaced on HPFF:

    ... and I thought that I'd ask if there is any progress or, indeed, if it's been written off as unbuildable.

    I have two Suomi drums waiting for conversion, but if it's not going to happen, I still have two Suomi drums -- which is not a bad thing!


    1. I've gotten some emails about it recently, so that isn't surprising.

      I have put it on hold for a while, pending finishing my degree. I'm close to a finished product, but had to shelve it due to senior year classes taking up all my spare time.