Jam/dry-fire free Ammo hopper concept

The standard approach for an ammo hopper is to have a big container and at the bottom have an outlet tube. Generally there will be some motors involved in moving the BB’s and it agitating the hopper to prevent the bb’s clogging against each other.
The advantage of this hopper is that it is (theoretically) simple and has a large ammunition capacity. However the necessity of agitation and extra feeding systems add complexity.

I’m currently attempting to design a simpler hopper. My aim is to create a design that cannot jam and still has a fairly large ammunition capacity for it’s volume.

I’m opening this thread to see if any of you guys with experience in hopper design will shoot down the idea straight away (maybe there’s something I’ve missed).

A simple jam-free hopper is a single tube with a spring at the end. However, for 250 rounds of ammunition you need a hopper 6mm x 1500mm - not likely to fit on your mech.

To fit in more BB’s, you can wind this tube into a spiral. Here are a couple of spirals each with 250 bb’s:

(all the units in the image should be mm rather than m, and I should have used proper dimension lines rather than just stretching the measuring tool by eye. Sorry). Probably the aspect ratio I’m happy with is a diameter of 45mm and a length of 70mm.

The problem with a spiral is that it’s hard to get a super long, super soft spring to go down the middle. Fortunately you don’t need a complete spiral, and can replace it with an outer-shell and an inner-shell. By rotating the inner shell, the BB’s can be made to move up or down the outer shell:
(Ignore the car-wheels-moving-backwards effect)

The rotation could be driven by a motor to allow rotating by exactly one bb size whenever you shoot, or it could be driven by a torsion spring (possibly a rubber band)

Essentially this is a recirculating ball-screw with the return channel going to the gun.

Possible further development:
In theory you could have an inner ball-race as well with the thread going the other direction. This would roughly double the capacity. Unfortunately the transition from the inner spring to the outer race to the inner race has to pass through the rotator. There probably is a solution for this but I haven’t managed to find one that didn’t require unmanufacturable parts.



  • No agitator required
  • Theoretically Can’t jam™
  • 3d printable
  • Operates in any orientation - upside down, on it’s side, etc.


  • Lower ammo capacity per volume. If the loader is large enough, you could possibly put other things inside - the only needed space is around the edges.
  • Pretty much only producible with 3D printer due to inside thread
  • Have to load it one-rotation at a time (about 20 shots), and it’s hard to check how many are remaining by eye.

Do you guys foresee any issues with this design? How many bb’s do you guys go through in a fight (is 250 going to be enough)?

Unfortunately I don’t have any 6mm bb’s on hand, and due to CV-19 I can’t get any for the next month (or until whenever the lockdown ends), so this will almost certainly remain conceptual for the near future. I just wanted to throw this idea past some people who have some actual experience to see if they think it is worth pursuing.

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250 or so should probably be sufficient, depends upon how trigger happy you are! A match might be 5-10 minutes, which gives you one shot every second or two on average.

I’m about to build up a new hopper myself, and was going to try and spin up @GhengisDhon’s design, but something along these lines also seems interesting. Fortunately, I do have plenty of BBs and printer time here in lockdown land.

Interesting concept. Off hand don’t see any issues why it shouldn’t work… but I’ve thought that of many of my designs that don’t end up working as expected.

250 shots will work. You just need to be deliberate about when you fire. For a light Mech, you don’t want to weigh yourself down anyway. The mech Nomad runs with around that number of rounds. Just need to remember to reload between rounds. With only 250 capacity, not much forgiveness if you forget. Nomad has lost a few matches because he forgot to reload.

If you want to try the concept, I would be glad to 3D print it for you (if you send me the STL fiels), give it a test run, and post the videos.

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I’m pretty sure I had mentioned this fully self-contained design on the trossen forums a while ago.

In theory, it holds 16x19 BBs in the external coil/thread surrounding the barrel with the outer shell pushing the BBs through the thread to the barrel. Loading is accomplished through the hole at the front (right end-cap), but with fixed ribs in the outer shell BBs could only be loaded as the shell rotates. Using only removable rib(s) would speed up loading and insure there are no missing BBs in the magazine. Had not touched this in months, so had not put much effort into the upper piston charging mechanism or the BB loading mechanism in the left-side end-cap before my interest was piqued again.

Wanting to start building this design is why I ordered a E3D v6 hot-end and ‘bulldog’ extruder today to hopefully cobble together a somewhat functional 3D printer. It’s one of those chicken-egg problems: want a custom printer, but need some custom printed jigs to assemble that printer. BuildOne is likely dead or maybe shipping sometime late this year, so I’m not as well equipped as I should be during the present work-free time.

I was thinking of @tician post that he quoted when I saw that! I think loading it would be a major PITA, but I also think it could work well when completed.

You could totally make that on a lathe with a boring bar, or perhaps on a mill with a long-reach form tool. The tool I use for my smaller spiral has 32mm reach, but maybe it could be cut from both sides, or made in segments. Bigger mills/tools and custom-ground tools could obviously do that better :slight_smile:

I hadn’t thought about computing the number of shots-per minute. I’m not anticipating being able to fire more than once a second or so due to gun limitations, so it seems like 250 should be enough,

Thanks for the offer of possibly constructing one! It’ll probably go through multiple paper/CAD iterations before I end up with anything actually worth printing, so it won’t be anytime soon.

Yup, that looks very similar. I don’t remember ever seeing that on the trossen forums. Does anyone have a link (was there anything else mentioned there that may be useful?)

I notice that your design has “ribs” on the outer shell for every individual BB, which means that you have to load them one at a time. I can’t see any real benefit to having more ribs? All it means is that most BB’s are pushed along by other BB’s rather than directly by the wall. Having only a single rib means that it’s loaded in groups of BB’s rather than one at a time. Having only one would possibly make it easier to have the rib sprung/removable.

Interesting idea to have the gun inside the tube. I’ll have to think about that - it does mean that the outside of the gun is the part that rotates which makes it harder to mount. I wonder if the ammo feeding could be mechanically coupled to the drawing back of the piston for true one-motor operation. Probably the mechanical complexity would make it not worthwhile overall.

A boring bar! cool! Seems I need to re-evaluate what a properly equipped workshop can do. (BTW Your machined aluminium over in the Malum thread looks absolutely beautiful)
And yeah, I think reloading is one of the biggest disadvantages of this style of ammo hopper.

It is still evolving, but was always intended to have a mounting bar/bracket floating over top that the gun is suspended from via screws at the front and rear end-caps. The extra ribs were mostly a symmetry thing at the start to insure every BB was being actively pushed into the breech regardless of where the last BB in the magazine is relative to the rib.

Haven’t started any threads/topics yet, but does the 10 day inactivity lockdown apply to every new topic?

The more I think about it, the more I remember. I was planning to have it directly mounted to a bracket on ripley’s elbow servo as if it were the forearm/wrist bracket of the HROS-1. The mounting bar serves to reinforce things in case of a fall.

For this forum, yes, once no reply has happened for 10 days, the thread will lock. New threads are OK!

This is a setting that can be changed easily – no idea whether the 10 days are reasonable or not. Happy to change it as appropriate.

Also, I haven’t yet attempted to crawl the Trossen Mech Warfare threads using some VBulletin crawler. That might be something to do, if that forum will keep falling off the internet, too.

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