The Miniature Bolt-Action Pneumatic Potato Gun – Part 2: Pneumatics

As mentioned in the last post, this may not be safe; I don’t recommend building this device.

pneumatics/ trigger viewBolt action rifles, as well as bolt-action potato guns can be loaded much faster than their muzzle-loaded counterparts.  In this case, the potato can be pre-cut and doesn’t have to be stuffed down the muzzle to a pre-determined depth.  One just has to insert the projectile into the breech as explained in this post, lock it into position, and fire.

Breech loading may seem interesting on it’s own merit, but it could still take a full minute or longer to fully charge your air reservoir to power the projectile down the barrel.  Without improved pneumatics, you still have a pretty slow process.  To speed this process up and give you several good shots between air charges, my device uses two separate air chambers.  One holds the main charge, and one holds the charge that actually fires your potato.  The following diagram will hopefully explain things somewhat:

pneumatic grape gun block diagram

Explaining this diagram: air is pumped into the main reservoir (1) by an air pump.  It then flows into the secondary air reservoir (3) through a flow control (2).  The secondary air reservoir is used to fire the projectile (5) by depressing the trigger (4).  Because of the flow control, when the secondary air reservoir is depleted during firing, it’s slowly filled again by the main air reservoir.  The charge in the secondary reservoir is a bit less every time, but one can still get several decent shots off before the gun has to be pumped up again.

grape gun assembled and numberedThe picture to the right is what this should look like when assembled:

To get into some of the details of everything, the air lines going from the secondary air reservoir (3) to the bolt mechanism (5) are 1/4 inch, the other line is 5/32 inch air line.

The flow control that I used in this device is made by a company called “Humphrey”, however, another good option (and probably more accessible to many people), would be those made by Automation Direct.  Also, Reid tool sells many pneumatic components or their equivalent that I used here (not that I’m recommending you try to build one.

The trigger mechanism is an air blowoff valve with 1/8 NPT taps on both sides.  The main air reservoir is a length of 2 inch PVC pipe and the secondary is a length of 1 inch pipe.  Shown here, one of the fittings is attached to the side of the 2 inch PVC pipe, but these fittings seem to give less leakage when drilled into the middle of the cap.

The air input is a typical Schraeder valve.  The one used here is meant for an automotive tire, which is installed using a simple through-hole.  These are also available in a 1/8 inch NPT form such as this one.

It’s a pretty simple setup really.  Not perfect, but it does allow several shots between pump sessions.  Some improvements on this would be a pressure regulator between tanks 1 and 3 so that a consistent (if slightly lower) pressure could be maintained for a couple of shots.  Also, both tanks should probably be larger to get more velocity out of the barrel.

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Here’s the final post in this series about putting everything together.

  1. Good work, Its nice and simple and gets the job done

  2. Very nice. May be do it for son )

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