(BASR)
Updated: 2/1/2006
The BASR is my first attempt at a homemade Nerf rifle. The primer for this project was actually a bet. I don`t remember the actual conditions of the bet but it was something like "build a high power nerf gun with shells". I agreed and proceeded to design. The resulting rifle is unique in every sense. The action is unlike any other in that is rotates and translates to expel the spent cartridge. Initially the propulsion was to be compressed air but that idea was abandoned when it became utterly apparent that sufficient pressure could not be attained for multiple shots with the size of tank used and the length of barrel. Instead, CO2 was implemented. The final rifle will shoot about 140 ft and is good for 20 rounds per 12 gram CO2 cylinder. A secondary fire mode was added to the rifle after completion. The interchangeable barrels that reside in the foregrip can shoot mega darts or burst of micro dartettes. The rifle then became a formidable ultra long range weapon as well as a last resort short range weapon.
The only this I was sure of at the beginning of the design phase of this rifle was that it was to use PVC shells. I brainstormed a few different ideas for actions. Initially I was thinking about a sprung magazine but I later just decided to use a gravity feed magazine for simplicity. Gravity feed meant the shells would enter from the top. Because of the bolt-action, a fixed ejector could not be used....too slow. I thought about a dynamic ejector, like a spring loaded one. I planned on that for a while but then decided it would be to complicated. The only option left for ejection was gravity. I would have to dump the shells out the bottom of the receiver and let them fall free. This meant I would have to feed the shell from the top and eject it through the bottom. The led to a rotating bolt system. The magazine was canted slightly to the right. This allows the used to look down the barrel and also reduced the angle that the bolt had to rotate. The ejection port was cut about 30 degrees to the right of the vertical. Ultimately the bolt had to rotate about 90 degrees for gravity ejection.
Ordinary PVC piping is the main material used in the construction of the BASR. The sizes used are as follows:
1 1/4" SCH 40 PVC (Receiver)
1" 200 psi PVC (Bolt)
1/2" PVC Couplers (Barrel spacers, Shell Rim, etc)
1/2" PVC (Barrel, Shells, etc)
Some other special PVC fittings were used in certain situations. For situations where a flat surface was required balsa sheeting was used. I use 1/8" sheeting and hand picked the hardest pieces for durability. Other materials could be substituted in place of the balsa, like lexan or another type of wood. I like balsa because it is easy to work worth, can be hard, and I have an abundance of it. To space the PVC pieces inside one another, I used electrical tape. This works extremely well and can produce extremely tight fits. I personally use CA glue (aka superglue) to due all the bonding involved. It adheres extremely well to both PVC and balsa. The advantage is the near instant drying time. CA will hold just as good as PVC cement. For assembly I used 1/2" #6 pan head sheet metal screws. These are just the right length to penetrate from the outside of the 1 1/4" PVC to the 1/2" PVC of the barrel but not penetrate the inner wall of the 1/2" PVC. They are very common and can be picked up at Walmart. The last major materials utilized in the FAR were music wire and brass tubing. These two were used in conjunction at all pivot points. Music wire was also utilized for retaining pins, pushrods, etc. See the materials list in the plans for a more detailed list of materials
Initially the rifle was to be powered by compressed air. I intended for the fore grip to house the tank. I fabricated a tank out of 1 1/4" PVC and rigged it to a spare RF20 pump I had.
The tank proved to be too small as it only held enough air for one good shot. It also took a phenomenal amount of pumps to achieve a decent pressure. When fired the dart would range about 30 feet maximum. The pressure would drop so much before the dart would leave the barrel that friction would take over and kill the darts momentum. The solution was a larger tank....much larger. Especially if I wanted multiple shots from one charging. Concepts including pack backs and external air supplies were brought up and quickly dismissed. I wanted the rifle to be easily carried but still pack a punch and be practical. Ultimately the air system was ruled unacceptable. An alternative would have to be used.
While there are other alternatives, like combustion, CO2 is the most practical in this situation. I intended to use standard 12 gram CO2 cylinders. These are very common and are used in a lot of pellet and low cost paintball markers. 12 gram CO2 cylinders nominally run around 900 psi before pierced. This was well above the burst pressure of even the smallest PVC. I had to figure out how to contain the pressure. I was quite sure I could not build anything out of homebrew materials that would contain that kind of pressure. I decided to use something that was specifically designed for it. A trip to Walmart produced a Brass Eagle Talon paintball marker. I intended to use the valve and CO2 containment system from it. The valve was easily extracted from the gun. Unfortunately the setup of the valve did not lend itself for easy use in my rifle. The output of the valve was down the valve shaft itself. The talon used a sprung weight that impact the valve shaft and released a burst of gas. The valve was positioned directly in front of the barrel. The trigger of the marker simply released the mass. That system was a little complicated for emulation in PVC. I decided to make the valve work directly. Meaning the trigger will depress the valve shaft directly with nothing in between. The first obstacle here was redirecting the gas. The gas could not be allowed to flow as it was designed. It would go right into the back of the trigger. It had to be rerouted to the back of the barrel. A T-joint and elbow joint for vinyl tubing were used to create a U-turn in the trigger shaft. The gas could now flow out and away from the trigger while keeping the trigger still inline with the valve shaft.
I used good ole JB weld to bond the pieces together. Initial tests produced good results. A quick discovery was that there was a substantial amount of force need to open the valve. The short trigger that I used first was way too short and required a lot of pull to fire. The trigger evolved many times growing length to ultimately be about 4 inches long.
Any problem as encountered. The quick expansion of the CO2 cooled the JB weld very quickly and it became very brittle. Add that too the large force on the structure and the JB weld eventually fractured. The solution to this problem was reinforcement. Brass tubing was placed inside the butt joints of the fittings. Larger brass tubing was then used to sheath the fittings at their joints. This provided cross over structures so that there was a more substantial connection between the fittings than just epoxy. This configuration has worked extremely well and has required no maintenance since its construction.
The final issue with the trigger involved the volume released for each trigger pull. The intended use for CO2 is short bursts of gas. In my configuration that valve was being held open by the shooter. Even the quickest hand would let way too much gas out in one shot. the average shots/cylinder of this configuration was 2 or 3. Unacceptable. I needed to implement a burst system of some kind. A system that would let a short burst of gas and then stop no matter how long the trigger was depressed. My solution was to install a cam on the trigger that would be timed with the triggers rotation. As the trigger is initially depressed the cam depresses the valve shaft. But as the trigger moves into its rearward position the valve shaft slips off the cam and stops gas flow. the result is a nice quick burst of gas and a much friendlier 20 to 30 shots/cylinder. The figure below shows the final fire control configuration.
Notice the long trigger for increased leverage. You can also see the cam on the back of the trigger very close to the pivot point. Brass was used to create a low friction spot on the valve shaft for the cam to slide on.
Early on I had decided on a gravity feed system and a gravity ejections system. This dictated the position of the feed and ejection ports. To align the bolt with the the appropriate port, the bolt has to rotate as it moves to the rear. The bolt is cut from 1" thin walled PVC. The PVC is cut with a twist to it to allow for twisting in the receiver. The cross section of the bolt at any location will be a semicircle. The section cut from the bolt is bonded to the inside of the bolt with a section cut from the rear. This creates a lip at the rear of the bolt. This lip is what grabs the shells and extracts it. You can see this in the figure below on the right hand side, just ahead of the sealing O-ring.
The bolt rides over the barrel. A turned down coupler caps the end of the barrel and rides on the inside of the bolt. This coupler is beveled to allow the shell to ride into place at the end of the barrel without catching. This can be seen in the left hand portion of the above figure.
The figure below shows the bolt and the receiver. They are positioned as they would be assembled when the bolt is closed. You can also clearly see the extraction lip in the bolt in this figure.
The shells are compressed of 1/2" PVC. Each shell is 3.5" long. The figure below gives a close up of a shell. A section of 1/2" coupler is bonded to the back of the shell. This rim is what engages the extraction lip on the bolt. This is the key to extraction.
The construction of the bolt-action rifle happened in pretty distinct phases
Phase 1
Here the action mechanism is complete and installed in the gun. The gun is complete but, no propulsion system is installed.
Phase
2A compressed air system was first tired to propel the darts. An air tank constructed from a 12" section of 1/1/2" PVC was hung under the receiver of the rifle. A small hand pump was used to fill the tank but had no place on the rifle . This setup proved to be inadequate to propel the darts to the ranges I wanted. With a crap load of priming you could shoot 4 darts maybe 20 feet. I didn't want to go any larger on the air tank/pump system so the idea got abandoned.
Phase
3The compressed air idea was abandoned and a CO2 system was installed. A Brass Eagle Talon was chosen as a donor for its CO2 valve. The system fit nicely in the grip of the gun and required some clever plumbing to get to work. A piece of 1/1/4" PVC was bonded under the receiver ahead of the trigger to give the gun some substance
Phase
4The compressed air idea was abandoned and a CO2 system was installed. A Brass Eagle Talon was chosen as a donor for its CO2 valve. The system fit nicely in the grip of the gun and required some clever plumbing to get to work. A piece of 1/1/4" PVC was bonded under the receiver ahead of the trigger to give the gun some substance
Phase
5After getting the rifle to the point where it fired reliably I realized just how accurate it was. The natural course to take advantage of this precision was to mount a scope. A trip to walmart provided a $10 Daisy air rifle scope. This scope is cheap but satisfies the requirements well. It has adjustments for windage/elevation to zero the rifle well enough to shoot can over from 20-30 feet away. The scope is mounted on a rail made out of PVC. I fabricated a mounting rail out of a sliver of 1" PVC and it is bonded to the top of 2 stacked 1/2" PVC sections. The scope is very solid on the gun....more solid that I personally expected.
Phase
6Phase 6 ushers in the improved cam trigger system. This produces a very quick burst of gas which allows for a vast improvement in shots/cyl. I now get about 20 good rounds before the cylinder starts to die out. This is acceptable as you are not going to be shooting people over and over. If you can hit your opponent square between the eyes on the first shot why do it again? The extended length of the trigger lever requires a much smaller force to fire a shot than before.
Phase 7
Phase 7 includes the addition of the secondary fire mode. A splice is placed in the gas line to the barrel. The split is routed to a threaded coupler below the barrel inside the foregrip. A small ball valve stops the gas flow into the secondary fire mode when it is not in use. To use the secondary fire you must chamber a plug round to block off flow to the main barrel. The valve is opened so and the trigger is pulled.
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All Image and Content © Evan Neblett 2006