A local Texan reached out to me the other day to see if he could help me with the development of my cartridge cannons. He's a fast gunner, but I'm not sure if he's on this forum & I don't know if he wants his name shared. I figure I would chronicle our testing. He has a pre-existing Admiral Sheer which is getting overhauled, so we are going to use her to fit the new alpha-test cannons. His requirements are 50-BBs, with 5-extra for tweaking. My existing prototypes hold around 22BBs; so today I increased the magazine size to roughly double. I'm curious how the increased magazine size does under pressure. I also want to reduce the barrel diameter, since it had such a big improvement on the 7/32" caliber cannon. More to come!!
Update! The first revision was only able to hold around 40rounds. After making the cannon taller, I decided I didn’t like it. I decided to fatten it up a bit, rather than make it too tall. Here’s the new 55-round BB cannon! I haven’t done pressure testing on it yet, so we’ll see.
Really cool idea! BTW, one way to improve the strength in the "Z" direction so that print layers don't delaminate is to just add threaded fasteners in the "Z" direction. In other words, print vertical holes sized to take a machine screw and thread them in to full height of your part. Small fasteners are more than strong enough... I like to use 4-40 or 2-56.
Update- the printer had a loose screw on the y-axis. The result was so subtle that I could barely tell, but things that were expected to be round were “not quite”. This caused major problems for the piston especially. I fixed the Y-wobble and am awaiting the next cannon this morning. I am also trying smaller diameter bores for the rifle. 0.19” almost worked, but it was just too tight without post processing. This morning I have 0.195” and 0.2” barrels waiting for me. Finally, even through Thursday’s cannon was a waste; it did not have any issue with the pressure test.
Todays print still has a problem with the piston. I must have unexpectedly changed some geometry. Today I’ll reprint the piston section and diagnose what happened.
On critical dimensions in my prints, I either oversize if I can afford it or drill or ream to size after printing. Having said that, I try to use metal parts for anything seeing static high pressure loads or that require high precision like seals. In a worst case scenario if you don't get it to work, you can always oversize the bore and sleeve it with metal tube.
I just realized, all my other posts for the cannon cartridge are on my Deutschland build thread. The cartridge has some good progress behind it. The only thing that’s different with this “alpha” cartridge is it’s scaled up to 55rounds. Original BB cannon, 25round cartridge: - This gun cycled about 200 rounds, flawlessly 1 BB per shot. https://youtube.com/shorts/eMr9UkasGoo?feature=share 7/32” Big Gun cannon, 15round cartridge. - This cannons still has an occasional magazine jam, but it’s ready to be scaled up. https://youtube.com/shorts/2cOmSpSXNqE?feature=share
thanks! I’m seeing really good luck with 0.5” threads & a built-in O-ring seal. Basically, the threaded plug “presses” into the ORing creating the air-seal. From there, just a simple acetone brush on the ABS to protect against leaks! The magazine walls under full 150psi pressure is holding, however there is a slight flex to it. I think over time this could be a failure mode. I’m moving forward, but also considering a small “support” cross-member to give the middle of the wall something to hold on to. ;(
Update - I figured out the geometry issue; very simple. The piston was just too loose in the chamber. I increased the diameter by “0.57 hairs”, and now it fits snug and still has great action. I’m going to cycle a couple hundred rounds through it tomorrow. If everything feels good, I’m going to print the batch & drop them in the mail!
Update - I ran out of CO2 & couldn’t complete the test!! The NTXBG club transferred to me one of our CO2 fill stations, so I should be able to get rolling again soon!!
I got me a small shop compressor for bulk cannon testing at 100-120PSI, then I switch over to CO2 once the design is validated at lower pressures. Then I got a fill station of my own, but I mostly keep the fill station for battling.
Alpha cannons are out of the printer!! Next step - Acetone bath, test fire, then put them in the mail!!
So these are fast gun versions eh? Don't get me wrong I love Kotori87's guns. I have them in my Lutzow. But I would love to take these babies for a spin. I think its great for the hobby to have different "suppliers" of BBs guns for new people coming into the hobby. Also, I know you are still in testing but would it be difficult for it to accept an 1/8" hose rather than 1/16" hose? I have 1/4" NPT to 1/8" QD fittings.
