BB machine gun
A BB Machine Gun (BBMG) is a device designed to shoot a large volume of small diameter ammo, 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 systems have been successfully built as portable units.
- A cloud chamber agitates the projectiles 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 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 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. It was designed by Gatorlangman AKA DavidVaini.
The performance of a BBMG, as with any gun, depends on specific characteristics. For BBMGs, perhaps the most important are;
- Air supply pressure
- Sustainability of the supply pressure
- Air flow from supply to barrel
- Barrel length
- The fit of the BBs in 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 air supply needs to be fairly substantial. For air supplies with a resevoir such as compressors this can mean CB ratios of 100:1 to 1000:1. Note that, for a BBMG, the "chamber volume" includes the volume of the compressor's reservoir.
The BBs should fit the barrel well enough to minimize leakage of air around the ammo (blowby) but not so tightly that friction is excessive.
The chambering and firing of BBs in a BBMG appears to be a coupled process. Usually, when a BB is passing through 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. The ROF for a gun would appear to be often very close to the reciprocal of the barrel transit time for a BB.
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.
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 Minigun which clocks in at 33~66 RPS.
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. The projectile speeds of around 250-400 Feet Per Second(FPS) are comparable to higher end Airsoft Electric Guns or 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 copper or 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.
Latke's BBMG: An excellent write-up on how to build a Vortex BBMG.