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Bs-12 Advanced Tactical Nerf Rifle

Semi-Automatic/Pump-Action Aluminum Magazine Fed Rifle

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#1 boltsniper

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Posted 08 March 2011 - 08:05 PM

Almost a year ago myself and three other guys from work started a competition to build nerf guns. The goals were simple and as follows:

- Must fire standard streamline nerf darts
- Shoot 10 rounds in 10 seconds.
- Hit the back of a car from 80 feet

These goals were intentionally vague so that we the objectives could be approached from many angles. For instance you could shoot 10 darts at once to meet the ROF goal, etc

The actual contest kind of fell through but myself and one other guy continued building. This is my result and he should be finishing his soon and I’m sure he’ll stick it up here as well.

This rifle is the 12th gun I have made, hence BS-12, and I am dubbing it the Advanced Tactical Rifle, or ATR. It’s essentially the same type of rifle as the BS-8 with improvements in design, materials and the addition of semi-automatic cycling.

1420585665-295592-1-DSC02426.jpg
1420585665-295592-2-DSC02443.jpg

From the start I intended to make this rifle semi-automatic capable. I say capable because I never intended for it to be solely automatic. My vision was for a pump-action rifle with a semi-automatic mode available. Since the semi-automatic action would run off of compressed gas of some sort, I didn’t want it to be solely dependent on an energy supply that may run out, so I wanted it to be manually operable as well. I also wanted this rifle to be much more durable. This meant utilizing tougher materials than PVC and polycarbonate. While I still used both of those materials, the bulk of the rifle is aluminum 6061. Being able to machine the parts to very tight tolerances also lead to better seals which made it perform better and also enabled the auto-cocking function.
The whole thing is screwed together and with most of them being countersunk. There are over 120 fasteners in the whole rifle. Only a few parts are bonded together…mainly the pistol grip.

It fires streamline clip-system standard nerf darts WITHOUT shells making this rifle much much more practical to use.

What resulted is a detachable magazine-fed, pump-action, spring-plunger rifle which is auto-loading when attached to an external gas supply.

Specs:






Caliber 0.5 in
Plunger Volume 4.91 in3
Plunger Stroke 4 in
Muzzle Velocity 190 ft/s
Overall Length 33 in
Magazine Capacity 12
Rate of Fire
Pump-Action 80 RPM
Semi-Automatic 120 RPM
Range 165 ft
Barrel Length 12 in
Propulsion Hybrid Impulse/Acceleration
Magazine Detachable Bottom Feed

Using a standard chronograph I was able to clock the muzzle velocity. The max I have seen has been 194ft/s but it varies based on the dart. The lowest I have seen is around 170ft/s. In comparison any stock nerf gun will shoot at 45-50 ft/s. Standard mods will get them to around 60ft/s.

BALLISTICS
During the design of this rifle, I decided to write some programs to simulate the firing of a dart. By doing this and knowing the muzzle velocity of the gun, I can accurately predict the range. In order to compute the drag on the dart from the air I did a combination of simple computational fluid dynamics (CFD) models and empirical tests. I found a good value for the drag coefficient of a streamline dart. Knowing this value the subsonic drag can be computed and ultimately the trajectory. At transonic speeds the drag increases greatly due to shock wave formation. To account for this I applied an exponential increase in the drag from 0.8 Mach up to 1.0 Mach. I did not bother to go above Mach 1.0 for numerous reasons. For one….let’s be serious…no one is firing a nerf dart at the speed of sound. Two…I doubt a nerf dart would survive that velocity. Three….it is impossible for a spring-plunger type gun to even fire a dart above Mach 1.0.

Anyways, the figure below shows the full range envelope for a streamline dart. From 0-90 degrees firing angle and 0-1100 ft/s muzzle velocity.

1420585665-295592-3-DartEnvelope.jpg

Knowing the muzzle velocity is 190ft/s for the ATR, the range vs firing angle can be plotted to determine the optimal firing angle.

1420585665-295592-4-BS12-FiringAngle.jpg

For completion, the plot of range vs muzzle velocity can be made as well. Notice the knuckle in the curve near the end as the transonic wave drag kicks in.

1420585665-295592-5-RangevsVelocity.jpg

Finally, the actual dart trajectory can be plotted.

1420585665-295592-6-BS12-Trajectory.jpg

Maximum attainable range for the ATR is 166 feet. This calculated value correlates well with actual shot data.

On a side note, notice the maximum range from the full envelope of just over 300ft. It is physically impossible to fire a stock nerf dart any farther than that.

These scripts were written in MatLab and I am thinking about porting them to Java to make an interactive applet.


TECHNICAL
As is typical, this project started with a solid model to layout the major components and determine the primary dimensions. The main portion of the rifle consists of sections of 1.5” OD aluminum 6061 tubing with a wall thickness of 0.125”. The sections are connected together with what I call “interstages”. These are machined from 1.5” 6061 stock. There are four major sections that make up the rifle. Starting from the aft: Main Spring and Auto-valve, Plunger, Bolt/Breach, and Barrel. There are respective interstages between theses four sections.

1420585665-295592-7-CAD_ISO.jpg

1420585665-295592-8-DSC02331.jpg

1420585665-295592-9-Exploded_Labeled.jpg

BARREL
The barrel is 6061 aluminum tubing with an inner diameter of 0.53” and a wall thickness of 0.04”. This ID is perfect for near-frictionless acceleration of a streamline dart with a nominal diameter of 0.5”. A 4” section of 17/32” brass was turned down slightly to fit into the aluminum barrel at the breach end and acts as a constriction for an initial impulse to the dart.

1420585665-295592-10-DSC02380.jpg

PLUNGER
The plunger is pretty standard. The plunger head is machined from 1.25” 6061 stock and has an O-ring around it for sealing. The plunger shaft is 0.375” aluminum bar stock. The shaft goes through the plunger head and a steel pin goes through both to secure them together.

The main spring is a McMaster 9637K21 with a constant of 146.3. The length used in the rifle is a 11” so the resulting stiffness is 3.75 lbs/in, giving a full draw compression of 33 lbs. The spring interfaces the plunger through a perch attached to the back of the plunger shaft. The perch also mounts the auto-valve activating pin.

1420585665-295592-11-DSC02404.jpg

BOLT
The bolt is unique to this design. It is machined from a solid piece of 1.25” 6061 stock and the forward end of the bolt matches the ID of the barrel. So a near perfect seal is made here without the use of an O-ring. About 0.75” back from the forward end of the bolt, the diameter necks down slightly to reduce friction when cycling. The aft end of the bolt forms a plunger head looking area with another O-ring. This head is what pushes the plunger back when the action is cycled. The O-ring is needed here because the chamber forward of the bolt head is where pressure is built to cycle the action in automatic mode. Corresponding O-rings are also found at the aft end of the barrel, where the bolt enters the barrel, and in the through holes for the operating rod above the barrel. A notch is cut in the front of the bolt at the bottom to let air in between the bolt and into the plunger cavity as the action cycles forward and the plunger cavity is opened up.

The operating rod rides in through holes in the interstages above the barrel and connects to the bolt head. An adapter connects to the operating rod at the forward end and either connects it directly to the pump handle or connects it to the action spring, which is a section of surgical tubing. Surgical tubing offered the right amount of stiffness and stretch.

The pump handle is a section of 2” 6061 tubing. I turned some slots into it to offer some positive grip and it works quite well. TO keep the pump handle from sliding on the receiver and scuffing the paint it rides on two Teflon runners attached to the fore-end.

1420585665-295592-12-DSC02389.jpg

1420585665-295592-13-DSC02349.jpg

MAGAZINE
The magazine is similar to the Nerf clip system mag except it is built up with aluminum. The spring is bent from 0.03” music wire

1420585665-295592-14-DSC02351.jpg

FIRE CONTROL
The trigger is pretty standard. It consists of an aluminum plate with a ramp at the aft end. The catch is made from 0.125” steel rod and is welded together for strength.

1420585665-295592-15-DSC02415.jpg

Edited by CaptainSlug, 20 November 2018 - 02:26 PM.

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#2 boltsniper

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Posted 08 March 2011 - 08:07 PM

AUTO VALVE
The auto-valve assembly sits at the back of the rifle on the underside of the receiver inside the stock. The actual air valve is a 3-way push-button valve that allows the bolt cavity to be open to atmosphere while the bolt is cycled forward to vent the trapped air. When activated compressed air is forced into the bolt cavity to retract the bolt and cycle the action. The heart of the auto-valve is the slide which activates the 3-way valve. The pin that protrudes from the rear section of the plunger interfaces with the slide.
Auto sequence
1. The rifle is fired and the plunger pin impacts the front of the autovalve slide and pushes it forward opening the air valve.
2. The bolt is retracted under air pressure
3. When the bolt (and plunger) reach full retraction the plunger pin hits the back side of the auto-valve slide and shuts the air valve off. The plunger is now cocked
4. The action spring (surgical tubing) pulls the bolt back into battery cycling a dart into the chamber
5. Ready to fire

Air is supplied to the rifle through a retractable air line connection. When not needed it can be folded back up into the stock so as to not get caught on anything. From the air line connection air flows to the 3-way valve through ¼” vinyl tubing. The same size tubing carries the air from the 3-way up through the pistol grip and into a fitting in the bottom of the bolt/barrel interstage.

1420585665-295593-1-DSC02372.jpg
1420585665-295593-2-DSC02482.jpg
1420585665-295593-3-DSC02419.jpg


VIDEOS

Demoing the Pump-action function
http://www.youtube.c...h?v=PecpzdBJPKM

Details of the fire control system
http://www.youtube.c...h?v=NUbjqgbV2TY

Testing/Demoing the Semi-auto function
http://www.youtube.c...h?v=eVNfWWy7_e8

Firing in semi-auto mode
http://www.youtube.c...h?v=TSEx7DPkQ7g

Firing some more in semi-auto mode
http://www.youtube.c...h?v=iY779Do6I_U

Walk around the rifle
http://www.youtube.c...h?v=AFtXuOr9APk

Semi-auto detail
http://www.youtube.c...h?v=-zuEMfpNvqY

Chrono shot clocked at 191.8 ft/s
http://www.youtube.c...h?v=-Uwl-zRl8vM

4 cups with 4 shots semi-auto (forgot to disconnect the pump handle so it moves with the bolt. )
http://www.youtube.c...h?v=1ZnwDtJ_llQ

5 cups with 4 shots semi-auto
http://www.youtube.c...h?v=3d0p3gubOHk


MORE PICTURES
1420585665-295593-4-DSC02443.jpg
1420585665-295593-5-DSC02432.jpg
1420585665-295593-6-DSC02438.jpg
1420585665-295593-7-DSC02462.jpg
1420585665-295593-8-DSC01764.jpg



I’ve left a ton out so feel free to ask questions. I can also post any specific pictures and videos if anyone wants more.

A few more pictures can be found in the photobucket album
http://s681.photobuc.../eneblett/BS12/

Edited by CaptainSlug, 20 November 2018 - 02:35 PM.

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#3 NerfGeek416

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Posted 08 March 2011 - 08:13 PM

WOW. :)
That just blew me away. That may be IS the most awesome thing I have ever seen on these forums.
WOW. That is just amazing.

Words cannot express my awe.
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#4 ChaosPropel

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Posted 08 March 2011 - 08:15 PM

Wow! You've done it again, Boltsniper! Great job with the build!
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Preheat oven to 350 degrees F (175 degrees C).

#5 Gears

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Posted 08 March 2011 - 08:32 PM

Dat lower engraving...

Edited by Gears, 23 February 2012 - 11:19 PM.

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#6 Cannonball

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Posted 08 March 2011 - 08:43 PM

That's just all kinds of awesome. Digging the ARs too.
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#7 landstricker

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Posted 08 March 2011 - 08:43 PM

That is outstanding, great job boltsniper. I love how people just keep pushing things up a notch.
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#8 Doom

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Posted 08 March 2011 - 08:54 PM

Extremely impressive work, as always. You've been very comprehensive so far, so I don't really have many questions about the gun, but I do still have a few questions.

I'd be interested in seeing your ballistics code as I'm pretty interested in that aspect of Nerf gun design. I'm under the impression that you only analyzed the exterior ballistics. Is that correct?

Also, why did you bother factoring in compressibility? No Nerf dart should be fired over about Mach 0.3. Assumption of incompressibility are perfectly accurate in my experience for exterior ballistics. Interior ballistics is a different story, however; the wave propagation process can be ignored for most Nerf guns but the compressibility effects on the properties of the gas can not be. (For some details of that, I have a very simplified non-dimensionalized model of the interior ballistics of a pneumatic gun described here. The springer model and any of my exterior ballistics stuff have not yet been posted.)

Three….it is impossible for a spring-plunger type gun to even fire a dart above Mach 1.0.


No, it's possible and it has been done before. A number of spring-plunger pellet guns fire in the supersonic range. Spring-plunger guns are actually a rather decent design for high speed ballistics because the adiabatic compression of the gas increases its temperature, which in turn increases the speed of sound, which in turn allows the gas to expand to refill the space behind the projectile faster, etc.
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#9 ricochet

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Posted 08 March 2011 - 09:18 PM

That is one gorgeous build, I must admit. I am very jealous of the fabrication tools you have access to in order to make the complex parts. You've inspired me before, and I think you've done it again. Keep up the fantastic work.
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#10 boltsniper

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Posted 08 March 2011 - 09:19 PM

Extremely impressive work, as always. You've been very comprehensive so far, so I don't really have many questions about the gun, but I do still have a few questions.

I'd be interested in seeing your ballistics code as I'm pretty interested in that aspect of Nerf gun design. I'm under the impression that you only analyzed the exterior ballistics. Is that correct?

Also, why did you bother factoring in compressibility? No Nerf dart should be fired over about Mach 0.3. Assumption of incompressibility are perfectly accurate in my experience for exterior ballistics. Interior ballistics is a different story, however; the wave propagation process can be ignored for most Nerf guns but the compressibility effects on the properties of the gas can not be. (For some details of that, I have a very simplified non-dimensionalized model of the interior ballistics of a pneumatic gun described here. The springer model and any of my exterior ballistics stuff have not yet been posted.)

Three….it is impossible for a spring-plunger type gun to even fire a dart above Mach 1.0.


No, it's possible and it has been done before. A number of spring-plunger pellet guns fire in the supersonic range. Spring-plunger guns are actually a rather decent design for high speed ballistics because the adiabatic compression of the gas increases its temperature, which in turn increases the speed of sound, which in turn allows the gas to expand to refill the space behind the projectile faster, etc.


Yes, the ballistics of the dart after it has left the barrel.

I did it just for completeness. Six years of fluid dynamics told me to... Needed an upper bound and Mach 1.0 is a round number. Also did it to show what the asymptotic limit in range is for a nerf dart.

I have gotten a nerf dart to Mach 0.52 before....

Yes, if you are talking pellet guns etc, it is possible to get an air based gun to fire greater than Mach 1.0. And you are right, it is because of adiabatic heating and other compressible properties that you alluded to. I was referring to a nerf dart specifically. You could probably get a dart over 1.0 due to the same reasons but it isn't going to be by much.
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#11 Doom

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Posted 08 March 2011 - 09:32 PM

Very interesting. I don't have a particularly strong background in compressible fluid dynamics (blame it on being a mechanical engineer, not an aerospace engineer) so I might have some specific questions in the future for you.

I meant more that there is no use in the hobby for darts over about Mach 0.3, unless someone is out to cause harm.

What drag coefficient and dart mass did you use for your simulations?

A while back I posted an equation I "derived" via curve fitting to a simple exterior ballistic simulation. How well does this fit with your simulation? (I seem to have lost the papers I used to go from the non-dimensional version that was the actual result of the curve fit, but I can redo that later when I have some free time.)

Edited by Doom, 16 February 2013 - 02:00 PM.

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#12 boltsniper

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Posted 08 March 2011 - 09:49 PM

I meant more that there is no use in the hobby for darts over about Mach 0.3, unless someone is out to cause harm.


Totally agree. I was just having some fun with the chrono a while back to see how fast I COULD get one.

Dart Wt: 0.045 oz
Cd is around 0.5
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#13 louiec3

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Posted 08 March 2011 - 09:51 PM

Is there any chance of getting detailed plans on this or the CAD files perhaps?
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#14 utahnerf

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Posted 08 March 2011 - 09:58 PM

YES. Great work as always Boltsniper, you've outdone yourself. You really did yourself a favor by using hex screws, they are soooooooo much better then Philips.

Edited by utahnerf, 08 March 2011 - 10:04 PM.

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#15 jwasko

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Posted 08 March 2011 - 10:15 PM

All intellectual conversation and amazing performance (by stock darts, no less) aside...

Nobody can turn a bunch of tubes into something as badass as Boltsniper can.




Edit: Yeah, it sounds awkward to me, too. Whatever.

Edited by jwasko, 08 March 2011 - 10:18 PM.

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#16 Forsaken angel24

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Posted 08 March 2011 - 10:24 PM

At first I was thinking this was some necro.
Then when I saw the date of the first post it felt like Christmas.
Then when I saw what you did it felt like I got everything I could have wanted and more.

Amazing work man.
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I don't get my kicks out of you,
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#17 Inferno Falcon

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Posted 08 March 2011 - 10:37 PM

Wait, it isn't Christmas?....Oh...

Fantastic work, BoltSniper, it's a shame the majority of the NIC does not have access to the tules required to make one of these. I'm guessing they're too much work for you to make a batch and sell them at a profit?
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#18 sodizzle3113

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Posted 08 March 2011 - 10:42 PM

I just watched all of the videos, twice, and I have never seen anything as sexy as that ever. Great, great, job on that.

Fantastic work, BoltSniper, it's a shame the majority of the NIC does not have access to the tules required to make one of these. I'm guessing they're too much work for you to make a batch and sell them at a profit?


I would guess so, it doesn't look cheap...
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#19 MrPzowned

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Posted 08 March 2011 - 10:44 PM

Boltsniper is the reason I nerf. Not kidding. I was 12 and saw the FAR and immediatley knew what my hobby was going to be.
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17:54 Noodle Yes what he did was stupid
17:54 Noodle but it's a plastic toy
17:54 Gears BUT IT COULD'VE BEEN MY PLASTIC TOY

#20 nix0101

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Posted 08 March 2011 - 11:03 PM

Wow man, amazing work and I am officially jealous of your workshop. I love how many people here are engineers and designers and use those skills to make and modify toys. If I survive the apocalypse I'm heading to your place for sure.
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Yeah. A length of rope, a good sturdy hook and a stool. Seriously, give your parents their lives back. -talio-

#21 Ozymandias

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Posted 08 March 2011 - 11:11 PM

It's Christmas somewhere. If it wasn't before, it certainly is now.

I am particularly impressed by the ballistic info.

If you had to give an educated guess, what is the cost of materials for your final version?

Are you sticking around for awhile?
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#22 Whisper101

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Posted 08 March 2011 - 11:31 PM

That beautiful, beautiful lathework make me want to shed tears of slightly jealousy twinged elation. I feel like this is what we've all been striving for, but you made it happen man. PM me if you get the chance, I'd love to talk about those turnings.
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#23 Carbon

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Posted 08 March 2011 - 11:32 PM

Holy crap, it's boltsniper! Great to see you back, and with such wonderful toys.

That's a substantial build, there. I'm gonna have to spend some time looking over this design...it's art, man.
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#24 KatanasPWN

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Posted 08 March 2011 - 11:56 PM

Boltsniper is the reason I nerf. Not kidding. I was 12 and saw the FAR and immediatley knew what my hobby was going to be.

Same thing happened to me, except I was like 8
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#25 shardbearer

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Posted 09 March 2011 - 05:37 AM

....


Wow.

If he was gonna sell these, it would probably be for ~$500. And people would buy them.

And if anything, Boltsniper is a reason for me not to nerf. When there is this, what is there left to contribute?

Edited by shardbearer, 09 March 2011 - 05:50 AM.

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