Airsoft Howitzer Air Cannon Help

[rescue341]

Hey all

Ive been working on a howitzer for a airsoft event but have ran in to a issue or two and im not sure what to try first. The cannon is a basic over under design with a .13 cu ft compression tank covered by a shrapnel shield (in case anything unfortunate happens) the tank is connected to a 1/2in ball valve then to the underside of the barrel. When I test fired up to 90PSI I was only getting about 20ft of range and some shots were not making it out of the barrel at all. In my mind this could be due to one of several things so i was looking for advice on what to try frist

1) this uses shells that are made out of 1-1/2in metal conduit with a hole in the side that sits inline with the air inlet in the barrel. The barrel is 2in but I was thinking since the air is entering the shell rather then the barrel this wouldn't be a issue. Perhaps I was wrong maybe the air is going out the back of the shell and around the gap between the shell and barrel?

2) I haven't finished the breach ejector yet so to test it I have just held my hand over the back to seal it. When fired I feel push back on my hand perhaps enough air is escaping to kill the range?

3) the shell is stuffed with cotton for wadding and then airsoft bbs. maybe the wadding isnt transferring the force to the BBs effently. (If the shells are the problem would any of yall have any ideals for firing BBs out of a 2in barrel)

4) maybe .13cu feet isnt enough air?

5)maybe 1/2in fittings are bottle necking the air dump?

attached is photos of the cannon any feed back would be very much appreciated

[jrrdw]

Put a spring on the valve lever to speed up the open time. For such lite wait ammo and a big bore you need to mass dump the air.

Use the GGDT to help predict the performance.

Welcome to Spudfiles! :bigsmurf:

[jackssmirkingrevenge]

I assumed some dimensions for a simulation but it definitely seems that for the pressure and barrel size you're using, your chamber is far, far too small even with a ultra fast valve:

By my calculations a 10 inch long barrel would give you better velocity for this chamber size and pressure...

Care to post a diagram of how exactly the shell fits in the breech?

the shell is stuffed with cotton for wadding and then airsoft bbs. maybe the wadding isnt transferring the force to the BBs effently. (If the shells are the problem would any of yall have any ideals for firing BBs out of a 2in barrel)

You want at the minimum to effectively use the pressure some sort of cup that fits snugly in the barrel to contain the BBs, backed up by a series of carton disks that are easily cut using a sharpened section of whatever pipe you're using for the barrel.

[rescue341]

I attached a badly done paint diagram. Forgive the awfulness. If I was an artist I wouldn't be going to college for electrical engineering haha. Basicly the shell is a half inch smaller then the barrle and just slides in against a stop ring (to keep the shell from becomeing the porjectile) In my mind the theory was the air rushes in though the hole in the shell that is inline with the air inlet and made the shell a mini barrel.

I have 8ft of 2in pipe left so I could make a larger chamber with out to much issue Could you recommend a size?

jhjbgyb.png (12.32 KiB) Viewed 3561 times

[jackssmirkingrevenge]

One bad paint diagram deserves another

usw.png (37.71 KiB) Viewed 3552 times

I would add a inner barrel of the same diameter as your cartridge, and make sure there isn't much of a gap between that and the 2 inch outer barrel to prevent air loss. That half inch gap on your current design definitely isn't helping you.

You need a bigger chamber and a faster valve, since you are using relatively low pressure I would go for at least 50% of your barrel volume as chamber volume, and consider a sprinkler valve modified for manual actuation as your firing valve, preferably the same diameter as your chamber.

[rescue341]

I've been playing with that software a bit today and according to it the biggest changes I can make seems to by making the valve and the plumbing lines bigger and putting in the inner barrel as you said. its also giving me a minor boast to double the tank size. But that may be me doing something wrong in the software. What i'm thinking I will do is replace all the plumbing coming out of the tank with 1 in. I will keep the 2 in barrel for ascetics but will have a 1.5in inner barrel and use 1-1/4 EMT for the shells maybe with some tape to make them tighter. And possibly double the tank size (I found out that doubling it would mean mounting the infiltrator to the barrel itself so i dont know if its worth it for the minor gain the software indicates?) What are yall's thoughts? Can anyone speculate on the kind of range with 370FPS? I was getting up to 20ish with 39fps Is range and FPS linear?


My present set up--

Changing the plumbing and adding a inner barrel

Doing the above plus doubling the tank

[jackssmirkingrevenge]

You're getting the hang of this

The best way to increase power is to increase pressure, not tank size. How are you filling it?

Can anyone speculate on the kind of range with 370FPS?

GGDT includes an external ballistics calculator, you'd have to tweak the numbers for an individual BB to get a realistic result but at about 370 fps, a 0.20g BB should go about 130 feet if the barrel is elevated at 5 degrees from horizontal. 20 degrees of elevation gives you 180 feet.

[rescue341]

this programs just full of handy surprises. You were just about on par the program spit out a range of 186ft. haha I am compressing the air with a 120v car inflator. I have a compressor on my vacuum former that i could temporally cannibalize but with this being made out of schd 40 PVC i think 110 is a good max psi. I pressure tested everything in the industrial lab at the college to 150psi. But were this will be used by people other then me I cant take as much risk with there safety as i can with mine. The field owner may just kill me if his insurance goes up haha

[jackssmirkingrevenge]

I am compressing the air with a 120v car inflator

These are devices that take forever to get up to pressure, if you don't want your howitzer rushed after it fires its first shot you're going to want as small a chamber as possible for relatively quick follow-up shots. This means optimizing your valve (1.5 or 2 inch sprinkler valve for example) to maximize airflow and maintain good performance without needing a large chamber.

[rescue341]

I accutley chose the infiltrator over something that would deliver instant air like a comperssor with a tank for two reasons the first is safety. I cant be at the cannon all the time for the 3 day event so Im not able to enforce a cool down time to let the PVC come back up to temp before firing the next shot. PVC may be more prone to failure if its fired while the chamber is cold. I knew going in that PVC was a bad material choice so I at least attempted to build in as much safety as possible The second reason is the forced cool down time allows the other team a chance to move up and keep the game interesting

[jackssmirkingrevenge]

I don't think your temperature concerns are an issue, but good point about forcing a cool own time to maintain fair gameplay.