SEMI-AUTO TECHNOLOGY DEMONSTRATOR
(STD)
Updated: 1/28/2006
Following the success of BS-1 and BS-2, if you call BS-2 a success, I got more ambitious and started looking looking at a semi-auto design. My ultimate goal was to build a magazine fed semi-automatic rifle. The brainstormed a few designs for an action and ended up going for a blow-forward design. To test the feasibility of the action I quickly built a working prototype. On top of the action I had to figure out how to build a sprung magazine. This became a parallel project. You may be familiar with the magazine, it is the magazine that I ended up using on the FAR. The action was built up and mated with a simple receiver and a temporary gravity feed magazine to test the mechanism. A CO2 system was used for power. The
A few designs for a auto loading action were brainstormed before one was chosen to pursue. I seriously considered a gas operated system very similar to current systems in modern firearms. For this system a gas port would be be placed at set distance down the barrel. The gun fires from a closed bolt with a shell chambered. As the dart passes the gas port in the barrel, gas is bled off and fed into an expansion chamber that drives the bolt back to cycle the action. I ruled this design out as I believed the pressure to be too low to cycle this kind of system.
The other option was basically the opposite of a gas operated system. Rather than using gas after the dart is fired to cycle the action, the action will be cycled before the dart is fired. This will ensure that the action is cycled for each shot. If there is insufficient pressure to operate the action, the dart will not fire. This kind of system can be compared to an open-bolt machine gun. In this kind of action the bolt is held open by the trigger sear when the gun is at rest. When the trigger is depressed the bolt is released. The bolt strips a round from the magazine, chambers it, and then momentum of the bolt traveling forward fires the cartridge. Gas is used to retract the bolt between shots. This type of operation is used in machine guns because of it's simplicity and it's high ROF capability.
An open-bolt system still uses expanding gases bled off the barrel to retract the bolt. As I stated earlier I was steering away from a gas port because of low pressure. Instead of gas to retract the bolt I decided to use elastic bands to pull the bolt to the rear.
The same shells as BS-1 takes were used for this gun as well. For the demonstrator the same magazine was borrowed from BS-1 just for testing. The figure below shows the demonstrator. Unfortunately, I did not take any pictures of the unit as it was during testing. A few pieces are missing in the images shown on this page. The ejector is missing along with a few mounts.
The bolt was non-rotating. For the demonstrator the magazine sat on top due to the gravity feed. The ejection port was inline with the magazine so the shells had to be kicked out almost vertically. To due this a dynamic ejector was utilized. The ejector itself was a triangular plate that pivoted on the black shaft visible on the underside of the receiver in the above picture. The section to the right of that black shaft is attached to the bolt and runs overtop of the ejector pushing it into the ejection port through a slot in the receiver and bolt. This action kicks the shell up and out of the receiver. The retraction bands were hooked around the ejector actuator and attached to the receiver at the triangular mounts on either side of the ejector slot. I can imagine this is hard to visualize with out actually seeing all the parts working together. The ejection system although a bit complicated worked well and reliably. The distance the shell was ejected depended on bolt velocity, but the nature of the system meant that the shell would be pushed out even if the bolt was moving very slow.
The image below shows the bolt removed from the receiver. The ejector actuator can be seen attached to the bottom of the bolt.
The piston in the rear of the bolt is the key to the system. the O-rings ride in a sleave inside of the receiver. The section to the right of the holes is plugged. When the trigger is depressed the gas pushes the piston forward. At the end of the cylinder there are bypass slots that allow airflow around the first O-ring. The gas is then allowed to pass through the holes and through the bolt to propel the dart. the second O-ring prevents the gas from escaping around the bolt. The bypass slots are barely visible in the image below.
The image below shows the bolt from a different angle. You can see the O-ring that seal the back of the shell when chambered. Very similar to the bolt in the BASR.
The shell extractor was borrowed from the BASR. The same concept of a second layer of PVC in the bolt creates a lip that engages the shell rim. In this case The PVC was bent near the end where the lip is created. This lifted the extractor lip off the bolt surface slightly and greatly increased it's engagement with the shell. In the above image you can see this just ahead of the sealing O-ring. The below image shows the extractor lip. The screws to the right attach the ejector actuator to the bolt and allow for disassembly.
As in the BASR the bold rides over the barrel. The barrel has an extension that is beveled on the inside to aid the shell when chambering.
The following two images show the bolt in it's fully retracted position and the closed position. In the image below you can see how the ejector actuator rides over the ejector pivot and kicks it up. The barrel extension is visible to the right.
The image below shows the bolt closed. A shell would be present between the bolt and the barrel.
Ordinary PVC piping is the main material used in the construction of the STD. The sizes used are as follows:
1 1/4" SCH 40 PVC (Receiver)
1" 200 psi PVC (Bolt, Piston Sleave)
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.
Once completion of the basic action, the system was tested for mechanical feasibility. A CO2 valve system was used just like the one used in the BASR. The gravity feed magazine from the BASR was borrowed for the testing. Four rounds were loaded into the magazine and the gun was fired. The first shot went about 10 feet but the action did cycle and eject the spent shell. I concluded that the poor range was due to the bypass ports being too small. These ports were opened up some and the gun was tested again. The range improved greatly to about 45 feet or so. Each shot was consistent and cycled flawlessly. So far so good.
The pressure from the CO2 was unregulated and was pretty high. Firing the gun got the bolt moving extremely fast and when it would chamber a round and seat it would really hit hard. the gun was somewhat hard to hold from this impact. I am actually surprised the shells were chambered without jamming with that kind of speed. I think the gravity feed helped with that because the shell was already sitting in the breach when the bolt comes along. If it had to strip it out of a magazine I think there would have been issues.
I fired the gun 10-15 more times just playing with it and I never had a failure to feed or failure to eject. Unfortunately, after about 20 rounds through the gun the impact of the bolt hitting home exceeded the strength of he PVC. The receiver failed around the magazine port. It failed during a shot and the front half of the receiver along with the entire bolt launched about 15 feet. The bolt broke at the bolt face when it landed.
That spelled the end of BS-3. About this time I was getting ready to graduate and move out of my apartment. I had no real free time to further develop and rebuild the gun. The action worked great and I think that the CO2 was just too much pressure. The action takes a fair amount of pressure to operate but it was just too high. I think it would have worked fine with 200 psi or so. Regulating the CO2 would have been the answer.
