Updated: 4/8/2006

NAVIGATION

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The BS-8 is a result of fielding the FAR for a year.  Several short comings to the FAR system were discovered and the BS-8 was designed to remedy these issues.  Like the FAR the BS-8 is a box magazine fed spring powered rifle.  The BS-8 was designed with a pump action rather than the fast action system.  The rifle is cycled by pumping the foregrip aft and then returning it to the forward position.  This rifle fires micro stefans from a brass smooth bore barrel.  The BS-8 is smaller than the FAR in all aspects making it much more agile of a weapon.  The technical designation of this rifle is BS-8 denoting that it is the 8th gun I have committed to the fabrication phase.  The descriptive designation is SCAR-N: Specialized Carbine Assault Rifle - Nerf.  This title was chosen for a variety of reason.  The obvious reasons are given by the acronym.  The term specialized means the rifle was designed for a specific tactical purpose, to be a superior frontline weapon.  The carbine term simply means the rifle is short at about 28" long.  The nerf term denotes the type of ammunition fired.  The US military issued a program entitled SCAR or Special Operations Forces Combat Assault Rifle.  This program was started to seek out a replacement for the M16 system.  The resulting rifles entered for trial built on the strong points of the M16 system but also remedied the short comings of the M16.  The BS-8 followed the same philosophy from the FAR so I thought the SCAR title was fitting.  The image below shows the SCAR-N with all major components labeled. 

SPECIFICATIONS

Description..........Carbine Assault Rifle

Designation.......................BS-8 Mk1

Release Date....................March 2006

Caliber..............0.54" (Micro Stefans)

Operation........Pump-action Rotating Bolt

Propulsion..................Spring Plunger

Magazine............Detachable Bottom Feed

Mag Capacity............................11

Overall Length.......................28 in

Barrel Length........................12 in

Max Rate of Fire...................150 RPM

Max Range...........................110 ft

Effective Range......................90 ft

Accurate Range.......................70 ft

Intended Use...............Primary, Sniper

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DESIGN

As mentioned above the BS-8 was designed to improve upon the FAR.  In order to do that, the short comings of the FAR had to be determined.  The FAR was released in March of 2005 and has been in service for a full year.  It has had probably over a thousand rounds through it in both indoor CQB and outdoor battle situations.  I came across a few issues with the FAR.  Some of them I anticipated from the start and some were unexpected.  The issues that I had with the FAR that I wished to remedy in the SCAR are as follows:

• The fast-action required the operator to remove the firing hand from the pistol grip to charge the weapon each time.  This takes extra time and causes the operator to regain composure between each shot. 

• The main unexpected issue with the FAR involved bolt bouncing.  When fired the plunger would travel forward and impact the rear of the bolt.  During this impact the entire bolt/plunger assembly would bounce backwards slightly.  This became an issue when the dart had not left the barrel when this happened.  When the bolt bounces back the pressure behind the dart is alleviated and there is no more propulsive force.  The dart exits the barrel with its own momentum at the penalty of barrel friction.  This impacts range.  Depending on the seal between the dart and barrel, the dart leaves the barrel and different times in the firing sequence and the impact on range is not consistent for each shot.  Some darts shoot 80’ and some shoot 50’.  

• The plunger is quite heavy causing lower acceleration values and lower ranges.  This was an unexpected issue.  Although it is intuitive that a lighter sprung mass will accelerate faster, it just didn`t occur to me in the design of the FAR. 

• The ½” PVC shells are quite larger and require a large magazine that still only holds 5 rounds.

• The ammunition for the FAR is mega stefans or stock micro darts.  These are good for stopping power but not as good for range and accuracy.

• Due mainly to the large shells, the length of the action, and the plunger stroke the FAR was a long rifle at about 36”.  It is not too long as to be unusable, but a shorter rifle would be easier to maneuver. 

• The trigger system, although it worked every time, was a bit spongy and had a large trigger pull.  Even with the buckle inhibitors in place the transfer bar would try to buckle causing the spongyness.  There was also not enough mechanical advantage to alleviate the strong force necessary to pull the catch down

• Although it has not failed, even though I am pretty rough with it, I have always been worried about the durability of the FAR.  The area around the mag well is pretty skimpy with lots of cuts and balsa holding it together.

As a result of these issues regarding the FAR, I set out to design the SCAR to address these points fro the get go.  The SCAR was to still be magazine fed through a detachable box sprung magazine.  It was to be spring/plunger powered and use shells for fast reliable cycling.  In a change from my previous guns I designed the SCAR fully in a CAD environment before ever fabricating components.  This led to minimal design-on-the-fly during fabrication.  There were still some changes made during fabrication based on materials available, etc.  The resulting design for the SCAR included the following design points to address the issues with the FAR design. 

• The rifle is pump-action.  This entails a foregrip that slides along the axis of the gun to cycle the weapon.  This is actuated by the support hand rather than the firing hand.  This means the operator need never remove his hand from the grip.  This also meant a higher ROF as there is no hand travel. 

• Some form of bolt lockup was needed to address the bolt bouncing.  The SCAR was designed with a rotating bolt lockup system.  This system is very common in many modern autoloading rifles.  The bolt rides in a bolt carrier and is interfaced with the carrier with a pin that rides in a track.  The bolt has lugs around its rim that engages a similar set of lugs on the end of the barrel.  As the bolt is moved forward the lugs align so that they may pass each other.  The bolt seats itself on the end of the shell the bolt carrier continues to move forward while the bolt remains stationary.  The cam pin rides along the track in the carrier and rotates the bolt to lock the lugs.  This process will be discussed further in the this article. 

• The plunger is made as light as possible.  Smaller/lighter materials are used in the construction of the SCAR plunger.  As a result the SCAR plunger weighs less than half that of the FAR plunger, at about 50 grams vs 120 grams. 

• Brass was used for the shells in the SCAR.  This would allow for much smaller shells to be used resulting in a smaller action and increased magazine capacity.

• Micro stefans are the primary ammunition for the SCAR.  Micros meant smaller shells still and also improved range and accuracy over the megas.

• As a result of the much smaller shells in conjunction with a shorter plunger stroke and shorter barrel, the SCAR is much shorter than the FAR at about 28”

• The trigger on the SCAR is similar in design to the trigger used on the GNS.  This type of linear pull trigger offers high mechanical advantage and is simple to fabricate

• The SCAR has minimal cuts in the receiver to increase strength.  Also, the receiver cover is SCH40 PVC rather than the thin walled making it very rigid and a good base for optics. 

Playing around with an AR15 magazine and a section of 17/32” brass a few years ago, I discovered that the brass will fit in the magazine quite well.  From the start I had planned on using an AR15 magazine for this rifle.  It made sense since it was readily available and worked well with the shells I planned to use.  This issue was how to fabricate a magazine well for the magazine and also a catch system.  Initially I thought of making one from scratch out of polycarbonate.  Then is occurred to me that I had already drawn it on CAD meaning I could easily enough just build it on the rapid prototyper.  Since the machine was doing the fabrication I could make accommodations for the stock magazine catch as well. 

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Ordinary PVC  piping is the main material used in the construction of the FAR.  The sizes used are as follows:

• 1 1/4" SCH 40 PVC    (Receiver)

• 1" 200 psi PVC    (Bolt)

• 1" SCH 40 PVC   (Receiver Cover, Stock)

• 1/2" PVC Couplers    (Barrel spacers, Shell Rim, etc)

• 1/2" PVC    (Barrel, Shells, etc)

• 1/2" CPVC   (Plunger Shaft)

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 SCAR 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

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The model was created in SolidWorks 2006.  Due to the nature of SW, you must have SW2006 or later to open the files.  You will not be able to open them with any version prior to 2006.  The drawings are image exports from SolidWorks and can be opened like any other image.  The materials list is done in MS Excel and is in .xls format. 

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Available for download are some videos showcasing specific points of the SCAR-N. 

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Nerfhaven Thread 1

NerfHQ Thread 1

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