Difference between revisions of "BB machine gun"

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A '''BB Machine Gun''' (BBMG) is a device designed to shoot large numbers of small diameter ammo, typically steel BBs or plastic Airsoft BBs. Most BBMGs are pneumatic guns. Their air consumption is much greater than a normal pneumatic spudgun. Most builders use a shop compressor as the air source making the BBMG less than portable. To be portable, a CO<sub>2</sub> tank and regulator can be used.  
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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 that portable, though models using CO<sub>2</sub> systems have been successfully built and used as portable units.
  
 
== Common Designs ==
 
== Common Designs ==

Revision as of 23:04, 31 May 2008

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 that portable, though models using CO2 systems have been successfully built and used as portable units.

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.
  • 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.
  • 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.

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;

  • 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 ratios 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.

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.

Typical Performance

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 Minigun, a modern Gattling machine gun, with a ROF of 33~66 RPS fires this fast. So, BBMGs are very impressive in terms of their ROFs.

The muzzle velocity of typical 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 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.

The very high ROF of most 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.

Most 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, BBMGs are typically fired in short bursts lasting a few seconds at most.

Links

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