Shrike - Airsoft Bow Fossil Works Projects

Named after the bird known for impaling its prey, the Shrike is the culmination of years of experience, along with a passion to drive technology forward in the sport of airsoft.

April 2016 - I produced a concept for a new design of "weapon" for use in the sport of airsoft - an action sport whereby players eliminate opponents by hitting each other with spherical, non-metallic pellets launched primarily by imitation firearms.

While most weapons are life-size replicas of existing firearms, I wanted to build something to turn people's heads that you wouldn't see in a normal magazine or catalogue.
Going back to the base principles of how most airsoft weapons work, I designed a system from the ground up, that utilised the limbs of a standard recurve bow to provide a force to compress the air used to propel my projectiles. This allowed me to use standard 6mm BB ammo in a weapon system that is nothing like anything else on the market.

I applied for Design Rights with the UK Intellectual Property Office and this was granted in August 2017 (Design Numbers 6016485, 6016486, 6016487). This was for the bow as a whole system, the attachment to a recurve bow that allows it to fire airsoft pellets, and the gravity-fed "Hop Unit" which enhances the capabilities and will be explained below.

Shrike Concept

Initial Ideas

My initial inspiration for this design came after watching my younger siblings playing with water pistols in the garden, or more specifically the type in which you pull back on a piston to fill the internal cylinder, and forcing this piston forwards shoots water out of the nozzle at the front.

This idea of using a comparatively long stroke piston made me wonder about turning an arrow into a piston, and firing that down a cylinder, using the compressed air from that to fire an airsoft pellet.

Proof Of Concept

I put together a simple prototype that afternoon, using a target shooting recurve that I previously used for field archery, some PVC pipe, and replacing the arrow point with a piston head largely comprised of epoxy putty and an O-ring along with some garden hose.
This was able to fire a muzzle loaded pellet well beyond the reaches of my garden. One unforseen benefit of this concept was the virtual silence in which it fires the projectile.

Airsoft weapons tend to use either electro-mechanical gearboxes to retract a piston, or compressed gas, such as propane or CO2 to fire pellets. These designs are inherently quite loud, producing either noise from the high speed motors used, or from the sudden expansion of gas as it exits the barrel, and the metallic firing mechanism is thrown to the rear of the weapon with each shot.

Functional Development

With the concept seemingly worth further investigation, I began to throw ideas around to make the system a useful tool. These included an inbuilt ammunition storage, to allow point-and-shoot use, rather than muzzle loading, as well as a mount that could be folded when not in use, to allow the user to sling the weapon for transport / to use another weapon in confined spaces.

Application of the Magnus Effect

The final major development in this concept was the application of the Magnus Effect - whereby spherical objects that spin backwards to their direction of motion experience a lift force. This would allow my projectile to have a much longer flight time, despite the comparatively low muzzle velocities (Approx. 100m/s - while firearms produce velocities in the region of 1000m/s). This increased flight time leads to a straighter travelling projectile over a greater range, temporarily countering the effect of gravity until the projectile slows down.

This brought with it some issues, however, such as how to apply backspin to the projectile, around a non mechanical, air-sealed hopper, which feeds a single pellet into the barrel for each release of the bowstring.

Eventually, I came up with the design shown above. This is unique to the sport of airsoft, as well as similar sports such as paintball or gel blasting, hence why I applied for specific intellectual property on this design.
This allows a single pellet at a time, to fall into the chamber from the hopper due to the geometry of the chamber, only one can fit in between the air intake (black), the hopper feed tube, and the rubber seal around the edge of the barrel (red). Around this hopper feed tube, a threaded nut (green) can move up and down, exerting a force on a pin, which in turn pushes a small amount of the rubber seal (red) into a port in the top of the barrel.
As air is forced through the air intake, this pushes the pellet in the chamber forwards, through the rubber seal, and into the barrel, whereby the seal applies friction to the top of the pellet at is heads down the smooth bore barrel, applying the Magnus effect as soon as the pellet exits the muzzle.
Remaining pellets in the feed tube are simultaneously pushed upwards, away from the chamber and back into the hopper due to the sudden increase in chamber pressure. As this pressure equalises, pellets begin to drop back down the feed tube, and one falls into the chamber, ready to fire again as soon as the piston is retracted.

Prototype Testing

Iterative Development

Prototypes 2 through to 7 have been tested in-game at various airsoft sites across the country.
Early prototypes were primarily tested at night-time games, at woodland sites, where the near-silence of the system offset the limited firepower and was actually a very effective tool.
This also quickly caught the attention of other players and became something of a local rumour, due the the lower attendance at night games, and the lack of a photographer.

When I eventually took my Mk.4 prototype to a daytime game at the same site, I received a vast amount of interest, from potential stakeholders that I hadn't considered. Aside from players looking for something unique, there is a whole branch of re-enactment and role-playing that was unbeknown to me, and were very eager to get their hands on my prototypes.
The picture seen right was captured at the first daytime game and this image quickly circulated the internet, ended up with over 25,000 views, and featured in an issue of Airsoft International magazine.

After around 80 hours of use across various prototypes I made a few changes including:

Reduction of cylinder bore to manage muzzle velocity and overall size.
Switching from a 40lb draw-weight bow to a 20lb Youth bow,
Use of an ambidextrous riser (grip of the bow).

Compliance

The Violent Crime Reduction Act 2006 has many stipulations around airsoft weapons. In the UK, airsoft weapons capable of firing multiple shots must have a muzzle energy lower than 1 Joule (with some exceptions). This translates to 100m/s with a 0.2g pellet, which is my upper limit on muzzle velocity.
This being a bow however, avoids many of these regulations as there is no way in which it could easily be mistaken as a real firearm of any kind.

Maintenance

While using these prototypes, it became clear that I would need to ensure that maintenance can be easily carried out. This ranges from cleaning the barrel, to the replacement of maintenance parts, like O-rings, screws and seals.

By the Mk.6 prototype (seen right), the system could be almost entirely disassembled into base components within 5 minutes, using only a 2.5mm hex key.

Photo by April Walsh - Striking Force Photography

Design For Manufacture & Assembly

Standardisation

While the early prototypes were made with whatever materials I could easily acquire at local hardware stores, prototypes 5 through to 7 have had a large emphasis on the use of standardised parts. This included use of one size and type of bolt used for multiple sub-assemblies, through to the use of existing airsoft components, such as AEG barrels, collars and hop-rubbers. This may slightly increase material cost, but does significantly reduce build time and complexity.

Manufacture

Certain parts cannot simply be bought off the shelf. For example, the hop chamber, which is a registered design of my on creation. For this, I have designed it in such a way that it can be manufactured from a series of sleeves and lugs. Minimising reliance on tight tolerances and reducing the number of machining processes needed through design.

Assembly

The Mk7 and onwards are designed to be easily disassembled, with minimal permanent fixtures and joints. The hop unit is held in place via a bolted bracket. Piston retained with spring pins.

The use of conventional retaining methods (for the airsoft industry) not only makes the system far easier to assemble / disassemble, but also more intuitive, presenting users with a familiar format for disassembly, despite the incredibly foreign appearance of the system.