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