BB machine gun

Revision as of 18:25, 2 June 2008 by Jimmy101 (talk | contribs) (Design Characteristics: seems we lost some sentences at the end of this section, added something so it ends logically.)

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 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
  • Air flow rate from the reservoir to the barrel
  • 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 first BB leaves the barrel the increase in air flow "chambers" the next BB. As a result it appears that there is rarely more than one BB in the barrel at a time. This suggests that the muzzle velocity and ROF of these types of BBMGs are linked. It appears that the ROF for a gun is often very close to the reciprocal of the barrel transit time. For example, a 3 foot barrel that fires at 300 FPS has an average barrel velocity of ~150 FPS and a transit time of ~20mS. The ROF would be expected to be about 1/20mS = 50 RPS. Design changes that alter the transit time would be expected to also change the ROF.

Typical 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($).

The other disadvantage of BBMG's is their portability and range. Portability can be fixed by using an external CO2 setup however that can get costly. The other problem with Airsoft BBMG's is their range. The range of BBMG's lack because of no hopup system or backspin created on the BB to keep it traveling straight. 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

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 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.

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 here). This muzzle velocity is about the same as a cheap commercial single shot 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 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.

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.


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