Difference between revisions of "BB machine gun"

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A '''BB Machine Gun''' (BBMG) is a device designed to shoot a large volume of small diameter [[ammo]], typically steel BBs or plastic airsoft BBs. Most BBMGs are pneumatic guns, with air consumption much greater than that of a normal [[pneumatic launcher]]. The common use of a large air [[compressor]] makes the majority of BBMG designs less then portable, though models using CO<sub>2</sub> systems have been successfully built and used as portable units.
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A '''BB Machine Gun''' (BBMG) is a device designed to shoot a large volume of small diameter [[ammunition]], typically plastic airsoft or steel BBs. BBMGs are pneumatic guns but unlike single shot [[pneumatic launcher|launchers]] they require a constant gas supply to function. The common use of a large air [[compressor]] restricts mobility for the majority of BBMGs although models using CO<sub>2</sub> or HPA systems have been successfully built as portable units.
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The basic principle of most BBMGs is the injection of pressurised gas into a chamber full of spherical ammunition, which is agitated and leaves the barrel accordingly.
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The principle can be seen in its most basic form, the "cloud" configuration, in [http://youtube.com/watch?v=_GMMbyJbUQM this 1000 frame per second video] by [http://www.spudfiles.com/forums/profile,mode,viewprofile,u,12312.html jor2daje]
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Variations of the position of the air inlet and the inclusion of vortex blocks typically aid with reliability and affect rate of fire, however muzzle velocity and energy is not affected.
  
 
== Common Designs ==
 
== Common Designs ==
  
* A [[cloud chamber]] agitates the projectiles by blowing the air stream up through them. The agitated BBs then get sucked into the [[barrel]].
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* A [[Cloud BBMG]] agitates the BBs in the ammunition chamber by means of the air inlet blowing upwards through them. The agitated BBs then get sucked into the [[barrel]] with the outgoing air stream.
  
* A [[vortex block]] blows the BBs around and around the inside of larg-ish diameter hole, which has a tangential hole leading to the barrel drilled in it.
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* A [[Tee vortex BBMG]] housed inside of the bottom of a [[tee]], uses the air inlet to swirl the BBs around a circular or semi-circular cutout in a block placed in the BB [[chamber]] and out through a BB sized hole in its side leading to the barrel.
  
* A [[vortex cap]] blows the BBs around similar to a vortex block but is used in smaller situations. Designed by Gatorlangman AKA DavidVaini the design incorporates a standard PVC or metal cap as the vortex wall. This results in a smaller design and can be incorporated into smaller projects.
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* An [[Inline Vortex BBMG]] uses the same [[vortex block]] from above however the block is housed inside a straight pipe. Normally Inline Vortex BBMG's have a spring/plunger system that forces the BB's into the vortex block to allow firing from any angle without agitation.
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* A [[Vortex cap BBMG]] works on an identical principle to a vortex block but instead of a cut-out in a block placed within a large BB chamber it uses the circular internals of a pipe endcap in which to swirl the BBs allowing for a smaller diameter chamber. This potentially allows for a smaller overall design as in [http://www.spudfiles.com/forums/rc-p240-vortex-cap-bbmg-p90-t12907.html this example]
  
 
== Design Characteristics ==
 
== Design Characteristics ==
  
The performance of a BBMG, as with any gun, depends on several characteristics of the gun. For BBMGs, perhaps the most important are;
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The performance of a BBMG, as with any gun, depends on specific characteristics.  
* Air reservoir [[pressure]]
 
* Air reservoir capacity
 
* Barrel length
 
* How well the ammo fits the barrel
 
  
Most of the characteristics that make a good pneumatic spudgun also apply to BBMGs. As with any pneumatic gun, the higher the reservoir pressure and the longer the barrel (up to a point) the greater the [[muzzle velocity]]. Because of the large amount of compressed gas that is wasted in a typical BBMG, the reservoir capacity needs to be fairly large and [[CB ratio]]s of 100:1 to 1000:1 are common. Note that, for a BBMG, the "[[chamber]] volume" includes the volume of the compressor's reservoir.
 
  
The ammo should fit the barrel well enough to minimize [[blowby]] (leakage of air around the ammo) but not so tightly that friction slows it down significantly as it moves through the barrel.
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'''Air supply [[pressure]] and flow'''
  
The chambering and firing of BBs in a BBMG appears to be coupled processes. Usually, when a BB is transiting the barrel, the reduced air flow through the gun inhibits the loading of the next BB into the barrel. When the preceding BB leaves the barrel the increase in air flow "chambers" the next round. This suggests that the muzzle velocity and ROF of these types of BBMGs are linked. Design changes that increase the muzzle velocity (for example by using a longer barrel) will tend to decrease the ROF.
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Most of the characteristics that make a good pneumatic spudgun also apply to BBMGs. As with any pneumatic gun, reservoir pressure is directly proportional to the [[muzzle velocity]].  As BB chambers tend to be fairly substantial in volume which is effectily [[dead space]], there is considerable pressure loss between the valve and the barrel as the process is analogous to a trying to pour water in a bucket with a hole in it.  This can be mitigated by:
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- using a high flow valve, forcing air into the chamber at a faster rate than is escaping through the barrel
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- using a [[detent]] to hold the projectile at the breech, blocking off the barrel and allowing pressure to build up before it is released
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- [http://i164.photobucket.com/albums/u28/hw97karbine/misc/bbstrafer.jpg storing the BBs directly in the pressure chamber], both passing through the valve meaning there is effectively no dead space
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- using a smaller BB chamber, though this means a corresponding reduction in ammunition capacity   
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'''Barrel tightness'''
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The BBs should fit the barrel well enough to minimize leakage of air around the projectile ([[blowby]]) but not so tightly that friction is excessive.  Even small differences in diameter can have significant effects on performance.  An attempt to quantify this power loss was made in [http://www.spudfiles.com/spudtech_archive/viewtopic.php?f=18&t=8544&hilit=4.5mm+5mm this experiment]
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Two 15 inch lengths of barrel tubing were used, one was aluminium with an I/D of 5mm, while the other was a straighted piece of coiled copper tubing with an I/D of 4.5mm. The barrels were hooked up to a blowgun attached to a compressor and used to fire lead BBs - with a nominal diameter of 4.5mm - at 100 psi through a chronograph.  Over a 5 shot string, the 5mm I/D barrel averaged 242 feet per second, whilst the 4.5mm I/D barrel averaged 279 feet per second.  With an average projectile weight of 8 grains, this gives a muzzle energy of 1.04 and 1.38 ft/lbs respectively - a significant power increase of 33% over the wider barrel.
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'''Barrel length'''
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The longer the barrel, the more time the BB is under the influence of gas pressure and therefore accelerates further.  Due to BBMGs usually fed with gas supply massively disproportionate to barrel volume, making normal pneumatic [[Chamber to barrel ratio]] concerns irrelevant. However, there does appear to be a relationship between barrel length and rate of fire in BBMGs, in that a fresh BB is usually not fed into the barrel before the previous BB escapes the muzzle, therefore a longer barrel would tend to reduce rate of fire.  If a detent is fitted at the breech, is is possible that the breech is blocked off by a following BB before the previous BB leaves the barrel, at best resulting in lower power and at worst leading to a collision inside the barrel which could lead to damage.
  
 
== Typical Performance ==
 
== Typical Performance ==
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The rate of fire for the average gun is very high, in the vicinity of 30 to 100 rounds per second (RPS)- 1800 to 6000 rounds per minute, comparable to that of a [http://en.wikipedia.org/wiki/Minigun modern gatling type machinegun].
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BBMGs in general are not usually capable of sustained full performance fire as most air sources cannot provide an adequate flow of sufficiently high pressure air. To reduce air consumption, maintain muzzle velocity and to reduce the rate that the ammunition is expended they are typically fired in short bursts lasting a few seconds at most.
  
 
'''Airsoft BBMG Performance'''
 
'''Airsoft BBMG Performance'''
  
Airsoft BBMGs are known for their high rate of fire (ROF). The ROF for Airsoft BBMGs is in the vicinity of 30 to 115 rounds per second (RPS), 1800 to 6900 rounds per minute. This extremely high ROF helps in Airsoft skirmishes for laying down suppressive fire or clearing out multiple targets up close. The Velocity of Airsoft BBMG's are actually quite good compared to Airsoft Electric Guns or AEG. The velocity of Airsoft BBMG's can vary anywhere from 250-400 Feet Per Second(FPS) which is in the range of higher end AEG's such as TM, CA or G&G. The advantage of the high ROF is quite clear in Airsoft Skirmishes however it is also a disadvantage. Since Airsoft BBMGs have such a high ROF they consume a lot of BB's which can get costly($).
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Airsoft BBMGs are known for their especially high rate of fire. This is in the region of 30 to 115 rounds per second (RPS), 1800 to 6900 rounds per minute. This is useful in Airsoft skirmishes for laying down suppressive fire or clearing out multiple targets up close. Typical projectile speeds of around 250-400 Feet Per Second(FPS) are comparable to higher end Airsoft Electric Guns (AEGs). The advantage of the high ROF is quite clear in skirmishes however the consumption of such a high volume of pellets can be expensive.
 
 
The other disadvantage of BBMG's are also their portability and range. Portability can be fixed by using an external co2 setup however that can get costly as well. The other problem with Airsoft BBMG's are their range. The range of BBMG's lack because of no hopup system or backspin created on the BB to keep it traveling further. This can be solved by adding a hopup system however they wear down quickly because of the high ROF. So as you can tell, the High ROF of Airsoft BBMG's has its advantages but also its disadvantages. A truly bitter sweet situation.
 
  
'''Copper/Steel BBMG Performance'''
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Other disadvantages of using BBMGs in airsoft lies in their portability and range. Portability can be fixed by using an external CO<sub>2</sub> setup for the gas supply however it can be costly. The other problem of range is due to the lack of a hop-up system which creates backspin on the BB and so increase the distance they can fly. Although possible to add a hop-up they wear down quickly because of the high ROF.
  
Copper/Steel BBMGs, even if very simply constructed from readily available materials, are capable of very high rates of fire (ROF). The ROF for typical guns is in the vicinity of 30 to 100 rounds per second (RPS), 1800 to 6000 rounds per minute. This rate of fire is much higher than military machine guns, which fire at ~9 RPS. Only the military [http://en.wikipedia.org/wiki/Minigun Minigun], a modern Gatling machine gun, with a ROF of 33~66 RPS fires this fast. So, BBMGs are very impressive in terms of their ROFs.
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'''Non-Airsoft BBMG Performance'''
  
The muzzle velocity of typical Copper/Steel BBMGs is much less impressive than their ROFs. With 3 foot length of 1/4" OD (3/16" ID) stainless steel tubing, a 120 PSIG 3 gallon shop compressor and firing standard copper/steel BBs, a typical vortex BBMG has a muzzle velocity of about 330 FPS (data from [http://www.inpharmix.com/jps/Jims_chrono.html here]). This muzzle velocity is about the same as a cheap commercial BB gun. The muzzle energy for an 0.177" copper/steel BB (0.33g) at 330 FPS is 1.2 foot-pounds (1.7J).Even though the muzzle velocity of most BBMGs is relatively low, the large number of BBs that strike the target in a short period of time can do a fair amount of damage.
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These BBMGs are unregulated by airsoft limitations of plastic pellets as they are not intended to be fired at other people.
  
The very high ROF of most Copper/Steel BBMGs consumes BBs at an alarming rate. A six pound container of copper/steel BBs costs about $15 and contains 6000 BBs. At 50 RPS that six pound container will last for two minutes of firing.
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With 3 foot length of 1/4" OD (3/16" ID) stainless steel tubing, a 120 PSIG 3 gallon shop compressor and firing standard steel BBs, a typical vortex BBMG has a muzzle velocity of about [http://www.inpharmix.com/jps/Jims_chrono.html 330 FPS]. This is about the same as a cheap commercial single shot BB gun. The muzzle energy for an 0.177" metal BB (0.33g) at 330 FPS is 1.2 foot-pounds (1.7J). Despite the relatively low projectile speed, the abrasive effects of a large number of hard BBs striking a target in a short period of time can be quite destructive.
  
Most Copper/Steel BBMGs are not capable of sustained firing since the air source cannot provide an adequate flow of air at sufficiently high pressure. To reduce air consumption (and hence maintain muzzle velocity), and to reduce the rate that the ammos is expended,Copper/Steel BBMGs are typically fired in short bursts lasting a few seconds at most.
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The high ROF can consume pellets at an expensive rate. A six pound container of 6000 metal BBs costs about $15. At 50 RPS that six pound container will last for two minutes of firing.
  
 
== Links ==
 
== Links ==
 
[http://www.burntlatke.com/bb.html Latke's BBMG]: An excellent write-up on how to build a Vortex BBMG.
 
[http://www.burntlatke.com/bb.html Latke's BBMG]: An excellent write-up on how to build a Vortex BBMG.
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[[Category:BB machine gun]]

Latest revision as of 14:25, 15 February 2011

A BB Machine Gun (BBMG) is a device designed to shoot a large volume of small diameter ammunition, typically plastic airsoft or steel BBs. BBMGs are pneumatic guns but unlike single shot launchers they require a constant gas supply to function. The common use of a large air compressor restricts mobility for the majority of BBMGs although models using CO2 or HPA systems have been successfully built as portable units.

The basic principle of most BBMGs is the injection of pressurised gas into a chamber full of spherical ammunition, which is agitated and leaves the barrel accordingly.

The principle can be seen in its most basic form, the "cloud" configuration, in this 1000 frame per second video by jor2daje

Variations of the position of the air inlet and the inclusion of vortex blocks typically aid with reliability and affect rate of fire, however muzzle velocity and energy is not affected.

Common Designs

  • A Cloud BBMG agitates the BBs in the ammunition chamber by means of the air inlet blowing upwards through them. The agitated BBs then get sucked into the barrel with the outgoing air stream.
  • A Tee vortex BBMG housed inside of the bottom of a tee, uses the air inlet to swirl the BBs around a circular or semi-circular cutout in a block placed in the BB chamber and out through a BB sized hole in its side leading to the barrel.
  • An Inline Vortex BBMG uses the same vortex block from above however the block is housed inside a straight pipe. Normally Inline Vortex BBMG's have a spring/plunger system that forces the BB's into the vortex block to allow firing from any angle without agitation.
  • A Vortex cap BBMG works on an identical principle to a vortex block but instead of a cut-out in a block placed within a large BB chamber it uses the circular internals of a pipe endcap in which to swirl the BBs allowing for a smaller diameter chamber. This potentially allows for a smaller overall design as in this example

Design Characteristics

The performance of a BBMG, as with any gun, depends on specific characteristics.


Air supply pressure and flow

Most of the characteristics that make a good pneumatic spudgun also apply to BBMGs. As with any pneumatic gun, reservoir pressure is directly proportional to the muzzle velocity. As BB chambers tend to be fairly substantial in volume which is effectily dead space, there is considerable pressure loss between the valve and the barrel as the process is analogous to a trying to pour water in a bucket with a hole in it. This can be mitigated by:

- using a high flow valve, forcing air into the chamber at a faster rate than is escaping through the barrel

- using a detent to hold the projectile at the breech, blocking off the barrel and allowing pressure to build up before it is released

- storing the BBs directly in the pressure chamber, both passing through the valve meaning there is effectively no dead space

- using a smaller BB chamber, though this means a corresponding reduction in ammunition capacity

Barrel tightness

The BBs should fit the barrel well enough to minimize leakage of air around the projectile (blowby) but not so tightly that friction is excessive. Even small differences in diameter can have significant effects on performance. An attempt to quantify this power loss was made in this experiment

Two 15 inch lengths of barrel tubing were used, one was aluminium with an I/D of 5mm, while the other was a straighted piece of coiled copper tubing with an I/D of 4.5mm. The barrels were hooked up to a blowgun attached to a compressor and used to fire lead BBs - with a nominal diameter of 4.5mm - at 100 psi through a chronograph. Over a 5 shot string, the 5mm I/D barrel averaged 242 feet per second, whilst the 4.5mm I/D barrel averaged 279 feet per second. With an average projectile weight of 8 grains, this gives a muzzle energy of 1.04 and 1.38 ft/lbs respectively - a significant power increase of 33% over the wider barrel.

Barrel length

The longer the barrel, the more time the BB is under the influence of gas pressure and therefore accelerates further. Due to BBMGs usually fed with gas supply massively disproportionate to barrel volume, making normal pneumatic Chamber to barrel ratio concerns irrelevant. However, there does appear to be a relationship between barrel length and rate of fire in BBMGs, in that a fresh BB is usually not fed into the barrel before the previous BB escapes the muzzle, therefore a longer barrel would tend to reduce rate of fire. If a detent is fitted at the breech, is is possible that the breech is blocked off by a following BB before the previous BB leaves the barrel, at best resulting in lower power and at worst leading to a collision inside the barrel which could lead to damage.

Typical Performance

The rate of fire for the average gun is very high, in the vicinity of 30 to 100 rounds per second (RPS)- 1800 to 6000 rounds per minute, comparable to that of a modern gatling type machinegun.

BBMGs in general are not usually capable of sustained full performance fire as most air sources cannot provide an adequate flow of sufficiently high pressure air. To reduce air consumption, maintain muzzle velocity and to reduce the rate that the ammunition is expended they are typically fired in short bursts lasting a few seconds at most.

Airsoft BBMG Performance

Airsoft BBMGs are known for their especially high rate of fire. This is in the region of 30 to 115 rounds per second (RPS), 1800 to 6900 rounds per minute. This is useful in Airsoft skirmishes for laying down suppressive fire or clearing out multiple targets up close. Typical projectile speeds of around 250-400 Feet Per Second(FPS) are comparable to higher end Airsoft Electric Guns (AEGs). The advantage of the high ROF is quite clear in skirmishes however the consumption of such a high volume of pellets can be expensive.

Other disadvantages of using BBMGs in airsoft lies in their portability and range. Portability can be fixed by using an external CO2 setup for the gas supply however it can be costly. The other problem of range is due to the lack of a hop-up system which creates backspin on the BB and so increase the distance they can fly. Although possible to add a hop-up they wear down quickly because of the high ROF.

Non-Airsoft BBMG Performance

These BBMGs are unregulated by airsoft limitations of plastic pellets as they are not intended to be fired at other people.

With 3 foot length of 1/4" OD (3/16" ID) stainless steel tubing, a 120 PSIG 3 gallon shop compressor and firing standard steel BBs, a typical vortex BBMG has a muzzle velocity of about 330 FPS. This is about the same as a cheap commercial single shot BB gun. The muzzle energy for an 0.177" metal BB (0.33g) at 330 FPS is 1.2 foot-pounds (1.7J). Despite the relatively low projectile speed, the abrasive effects of a large number of hard BBs striking a target in a short period of time can be quite destructive.

The high ROF can consume pellets at an expensive rate. A six pound container of 6000 metal BBs costs about $15. At 50 RPS that six pound container will last for two minutes of firing.

Links

Latke's BBMG: An excellent write-up on how to build a Vortex BBMG.