Question regarding spigot mortar style ammo.

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Alex345
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Tue Apr 14, 2020 6:13 am

So a while ago i built a fully steel, 1.5" piston-valved cannon with the intention of firing 28mm fin stabilized projectiles consisting of wire rods and washers. I was expecting some serious recoil, so i built a piston into the stock to help absorb some of that recoil. But JSR told me that the very heavy weight of the gun would already be enough to absorb the recoil, and he was right! Plus the fins proved too difficult to produce with consistency, with my current tools.

So after watching Gun Jesus doing a video on the Panzerbüchse 39, and how some were later converted to fire huge rifle grenades. I noticed that the 50cm long 1.5" steel pipe that houses the 28mm aluminum barrel is PERFECT for this purpose as i found a size of thick PVC pipe that fits over that pipe almost perfectly.

I have some questions tho:

What would be the easiest, most reliable and consistent way to mount any type of fins to a PVC pipe? I wanted to find a piece of hexagonal aluminum tubing that slides over the body of the projectile and then simply thread/tap it so that i could mount aluminum fins to each flat side of the hexagon. Like this:

Second: I see that this design would create A LOT of dead space, as the air would have to fill up the entire inside of the barrel first. Would adding a wooden dowel in the center of the projectile increase the speed? And if so, would the added weight and complexity be worth it? I also thought about adding a small projectile in the barrel, which would get launched first and smack into the main projectile. But I'm doubtful that the PVC pipe would survive that.
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Anatine Duo
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Wed Apr 15, 2020 6:28 pm

I like your thinking. I fooled around with the dowel inside the hollow projectile idea too, but gave up because I am lazy.
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mark.f
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Wed Apr 15, 2020 10:14 pm

Not only do you have the dead space to contend with, but the pressure will be acting on a smaller surface area than it would with a barrel which fit the same sized projectile (if that makes sense), so you would almost always be better served (performance-wise) making a barrel larger than your projectile. A conventional spigot mortar also doesn't have that "dead space" since the propellant is contained in the projectile and the "spigot" can be a solid hunk of steel.

It would be neat to play around with though.
Alex345
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Fri Apr 17, 2020 10:14 am

Anatine Duo wrote:
Wed Apr 15, 2020 6:28 pm
I like your thinking. I fooled around with the dowel inside the hollow projectile idea too, but gave up because I am lazy.
You could even replace the dowel with water... Tho that might really over complicate things hehe.
mark.f wrote:
Wed Apr 15, 2020 10:14 pm
It would be neat to play around with though.
Mostly this! I looked around in my work space, and this is one of the only things i could build with the stuff i have on hand without having to order things or visit a DIY store (Our gov recently decided to open them again on weekends under strict conditions, so i'ts gonna be a madhouse there...)

I once built a small 3/4" QEV gun with a very long barrel for which i made a novelty PVC spigot style grenade. It actually flew quite far considering the relatively low pressure and slow opening time of the valve (Was using a CO2 extinguisher squeeze valve to pilot it) I doubt this cannon will be very practical, but i'm sure it will be fun hehe
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Anatine Duo
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Sun Apr 19, 2020 3:16 pm

I have been thinking about hollow spigot launchers more now... was it this thread that caused it or is it the Umarex Air Saber? For the arrow launchers do they not worry about the dead space? Maybe not since they are hunting-oriented.

BTW fins don't need to be perfect especially on long projectiles. Feathers held on with thread and goo work well if the launch can be smooth.

My gut tells me the hollow spigot offers a cleaner launch than the Airbolt as well as being more streamlined, so perhaps gluing a dowel inside the arrow to take up dead space is worth it. Hmmmn...

I tried converting a Crosman Backpacker to spigot launching but the dead space is extreme (.22 ID and 7/16 OD) and the volume of the firing valve tiny so it was toylike compared to just shooting .22 dowels out of it.

Off to try and find reviews of the Airbolt.
KannonKing
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Fri Apr 24, 2020 10:48 am

I made super simple fins, and used a bottle cap wadding, a perfect fit for 1 9/16" barrel!


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Alex345
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Fri Apr 24, 2020 11:51 am

Anatine Duo wrote:
Sun Apr 19, 2020 3:16 pm
I have been thinking about hollow spigot launchers more now... was it this thread that caused it or is it the Umarex Air Saber? For the arrow launchers do they not worry about the dead space? Maybe not since they are hunting-oriented.
I'm guessing because the arrows are probably made to be very light, and PCP airguns pack quite a punch. I think the FX Ranchero Arrow uses the spigot style that I'm going for with this build. Tho the company i work for has opened again, so much less free time now (well, plenty of time after work, but yeah, motivation hehe).
Anatine Duo wrote:
Sun Apr 19, 2020 3:16 pm
BTW fins don't need to be perfect especially on long projectiles. Feathers held on with thread and goo work well if the launch can be smooth.
I've glued 4x 5cm long pieces of square aluminum profiles to the body of the projectile. Drying as i'm typing this. The cannon itself should be done (I've built it before). Just needs a stock or mounting pad and something to absorb the recoil, which i think will be quite bad. Will post some pics when the epoxy has cured.
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jackssmirkingrevenge
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Thu May 14, 2020 8:46 am

I pondered a Fly-K style type spigot mortar a while back...

edit: damn, that was almost 10 years ago. Time's a wastin'
hectmarr wrote:You have to make many weapons, because this field is long and short life
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