My first test is 30 min in a small metal trash can with soaked paper attached to can sides with magnates. I used a metal soup can to keep it off the bottom of the container. I am hoping the coarse 1/2” threads will survive; I’ll let you know once it’s dry by tomorrow!
Update, the 1/2" threads are looking good this morning! I'll try pressure tests tonight to see if 30 min vapor is enough for my printer!
It still leaked. I'm thinking I'll move to @Kotori87's method next. I figured it'll be faster to get to my answer of "is this possible".
Update - It leaked less but most critically it’s leaking between the cannons. Since i can’t think of a good way to seal between the cannons, I’m going to take this as a sign to pivot my concept. Tonight, I’m going to draw out “cannon cartridges”. One cannon per cartridge, and you stack as many cannons as you want. So if you want twins you stack two, for trips three cartridges.
OK. New Cannon Canisters mk1 are designed & ready for the printer. Now, if I had only not kicked off the final print of the SMS Deutschland hull; I would have a new cannon to try out tonight!
Update - I saw some micro cracks along the side of the cartridge where the threads are. I've increase the width of the cartridge to 11/16" wide. Total dimensions of the cartridge is 3.14" x 11/16" for an estimated >75-round cannon. I intend to reduce the height to get exactly 75-rounds. Most of the size is for the magazine; so I'll play around with shorter 25-round versions as well. I was seeing multiple rounds go through per shot; I printed out just the piston section to diagnose the issue. After acetone sealing it; I found that too much air was escaping from the "to magazine" airline. This was causing the piston to not have the holding strength that it needed. Making the hole smaller isn't a great option because of the printer limitations. Plugging that air line seems to get me a good amount of air flow around the piston; so I've elected to delete it.
Update - This revision reduced the number of spurt shots to a consistent 3-4 rounds no matter how long I held the gas open. I believe this to mean the interrupter is working, however the spacing where the balls can rest in the chamber is so large that 3-4 balls can enter the chamber at rest. I've made this area tighter tolerance and created a dimensional ball lock. I expect that the second round will now be unable to get past the first round, until the chamber is clear. I am having trouble with the breach o-ring holding the right amount of pressure against the barrel. I am seeing gas escape through the threads at the barrel and the magazine, however the chamber bolt is pretty air tight. This revision is tighter tolerance around the breach o-ring. The magazine will now use the same size o-ring as the chamber. I'll add thread tape to all threads this time to assist the o-rings. I also reduced the size significantly to reduce plastic cost during R&D. The foot print is now 2" x 2-3/16" x 11/16" I'm hoping for 25 rounds. This was allowing 3-4 balls to enter the chamber while at rest:
How were the rounds getting past the magnet on the bottom? At least, I assume that's a slot for a magnet on the bottom. The magnet should be catching and holding the first shot to enter the breech. That has worked quite well for all of my Fast Gun style cannons, even the funky designs I've tried. I don't see how scaling up the design for larger 7/32" rounds would mess with that.
LOL. I have no idea. I’m just guessing, but the theory fits the symptoms. That said, it’s scaled up; but I’m still primarily feeding it BBs. I think the channel was wide enough that multiple worked themselves in. I tried feeding it 7/32s, and it fired 2-3 extra instead of 3-4. I then tried 1/4, but they just got stuck.
It works! This was a full pressure test, with O-Rings tweaked down tight. The little cannon really impressed me this morning. It looks and sounds like it's hitting very nicely; it certainly hits plenty hard for Big Gun. I'm not really sure how to quantify it in fast gun terms. Is there something I can shoot at to compare to a fast gun cannon? Remaining issues: The magazine doesn’t consistently load. The tubes are too tight tolerance, so I had to ram a 3/16 bearing through it to open it up a bit. https://youtube.com/shorts/eMr9UkasGoo?feature=share
Today's revision is an A+B test. The first design is simply to make the mouth of the magazine a more aggressive drop plus filets to remove any sharp edges. The second design adds an air escape at the top of the magazine toward the barrel. My hope is that the air will rush through the magazine, knocking balls out of the way. Once the air starts escaping through the barrel, I'm hoping it'll create negative-pressure on the BB and suck it up to the O-Ring.
Update - The modified feed tube reduced the frequency of a magazine jam; however didn't eliminate it. The air escape had no effect, and jammed about the same frequency as the modified feed tube. - As an experiment, I waited for a jam to occur then plugged the rifle & looked through the magazine bolt. I did not see any ball movement from the air blow back. I'm investing my time in to further refining the magazine geometry, new upgrades: I've tried to make any ball-lock geometries to NOT occur at multiples of the ball diameter. I figure it'll be much more likely to get a ball-lock if it can occur when the balls are at rest. I narrowed the magazine width to size of the feed-tube, trying to make sure any ball-lock geometry doesn't happen in three dimensions.
Do you have a paintball/airsoft chronograph? That'll tell you how fast the rounds are going. Big Gun uses a foam penetration test to limit velocity, although the foams are not consistent between clubs, or even from year to year. It's kind of a problem. Back in the late 90's/early 2000's, someone in WWCC experimentally determined maximum velocities for the different Big Gun calibers using a chronograph and the foam that was available at the time. There was a table in the really old rulebooks for it. That original foam is no longer available so I cannot verify the results myself, but I do have the velocities from that table. I have NOT validated these numbers myself, but they give reasonably correct results. 1/4" rounds go up to 165FPS. 7/32" rounds go up to 185FPS. 3/16" rounds go up to 200FPS. .177" rounds go up to 220FPS. To compare to Fast Gun cannons, I've chrony'd my 3d-printed cannons at about 220FPS with the 120PSI output from my shop air compressor. I can get 240FPS or more from 150PSI CO2 in combat, but I don't often tweak that hard because they sometimes double-fire. I'm sure other people can post their velocity numbers. That said, velocity isn't everything. The bigger rounds have better penetration, despite the lower velocity. This is particularly noticeable against heavy armor, deep below the waterline, or at shallow angles.
This is very helpful. I don't have a chronograph, and would prefer spending $150 on it... That said; perhaps I can use a high-speed video on my phone to against a ruler backdrop. Assuming the camera is fast enough, it should get me the roughly the data I need...
I knew this to be true... however this comment sparked a thought; please check my logic? Pressure is built up when the BB plugs the O-Ring. When the pressure reaches the point where the BB goes past the O-Ring, the flow rate is what pushes the BB through the barrel where it builds up its speed. The metal fast gun cannons are limited in the air they contain. Once the BB leaves the restrictor, they rely on pressure coming from the pre-valve accumulator which means the air needs to flow through the valve/tubes/etc; which creates drag. For a 3D printed gun, the gun contains more internal air, which should make it into its own accumulator? The bigger the magazine, the more air is available locally. If a 3D printed gun can handle the 150PSI pressure with no leaks, then it should be able to match the performance of a metal fast gun. However, with more air local to the gun; doesn't that mean the 3D printed gun can theoretically have higher flow rate and hit harder?
Bleh, cannons are complex and other folks don't like my math. For Fast Gun cannons, I do not believe that flow rate has a significant impact on velocity. I've certainly seen some HOT cannons that have oodles of flowrate. But I've also seen some really hot cannons fed by MAVs and the smaller 1/8" OD gas lines. That's because it's pressure behind the bb that actually pushes it down the barrel. Flowrate and and internal volume help maintain that pressure. As the bb travels, the CO2 expands which causes pressure to drop, reducing the force on the bb. The more internal volume your cannon has, the less the pressure drops as the round travels. There are diminishing returns on internal volume though. At some point, there's basically no drop in pressure over the length of the barrel, so adding volume after that provides no benefit. You can supplement a low internal volume with a gas line blowing down the barrel (or other flow-boosting tricks people do on their high-performance cannons), but for most cannons, most of the performance comes from the gas already inside. A higher flow-rate mostly helps the cannon pressurize faster, so you can achieve higher rates of fire. If your flow rate is low enough, there may even be a noticeable delay between pulling the trigger and the cannon actually firing. Even with such a delay, the cannon can still hit hard due to its internal volume. Also remember that other features like longer, tight-tolerance barrels have huge effect on velocity by reducing blow-by and giving more distance for the pressure to push along. I believe that my 3d-printed cannons double-fire at their hardest tweak because the pressure inside the cannon equalizes across the piston, and then the spring pushes the piston down which allows the next round in. I am quite happy any time my bbs are moving 200fps or more, because that's plenty to cause damage in almost every condition I find my ship in. View: https://www.amazon.com/Caldwell-720001-Ballistic-Precision-Chronograph/dp/B00HTN5DTE/ Also, I just searched for airsoft chronograph on Amazon and found this. It's a huge step up over my 7-year-old chrony, and cheaper to boot. I'm ordering one.
$50 Chrono X310 Chronograph Xcortech X310 Mini Handheld Airsoft Chronograph, Accessories & Parts, Chronographs - Evike.com Airsoft Superstore Chrono's can be found for $25-$35+ on ebay and AliExpress...
Fantastic news!! I just fed over 200 rounds through it without a single jam. ...I would have burned through the entire can of BBs, but I ran out of CO2! Notes: - It fired as quickly as I could mash down my finger. - It fired exactly one ball with complete consistency. - The hit power seemed pretty consistent, but I ran out of gas before I could measure the ball speed. - I've put maybe 500 rounds through the little plastic piston; and I don't see any wear on it at all. The trick I found was to put a shelf on the magazine. - My observation in previous designs was the most likely jam was from the downward pressured BBs coming in contact with the balls on the ramp. - I figured a shelf would reduce the amount of gravity pressure at the choke-point. - It's also entirely possible that I lucked into some geometric advantage too... My next step is to secure some 7/32 bearings & scale this thing UP!
