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An airsoft gun has two main components that a bb travels through before leaving the gun. First, the bb must pass through the hop-up chamber, then down the length of the inner barrel.
Stock, or the original manufacturer's, inner barrels on an AEG have an inner diameter between 6.08mm to 6.10mm on average. Cheaper brands will have less precise barrels made out of cheaper and/or rougher materials.
Tightbore barrels are barrels that have a smaller inner diameter than the stock barrels The inner diameter typically ranges from 6.01mm to 6.05mm. The smaller diameter means that the bb has less room to float around as it travels down the tube, but also there is less room for air to squeeze by the bb. Thus there is an increase in air pressure, giving the bb a slightly higher velocity (fps) than with larger bores. Additionally, the tighter bore means that the bb will leave the gun in a tighter aim, or more accurately.
NOTE: ONLY HIGH QUALITY BB'S ARE TO BE USED WITH TIGHTBORE BARRELS!
NEVER RE-USE BB'S. PERIOD.
Using bb's that have been fired or picked up off the ground will tear up your inner barrel even if there are no visible flaws on the bb.
From what was discussed, tighter = better. Not always.
Airsoft guns are still made to be fired in the field, where dirt and dust are prevalent. If dirt gets in the barrel or on the bb's before they are fired, then the bb is more likely to jam in the barrel. Having a larger inner diameter gives a larger margin of error, or room for the dirt so that the bb can still pass through the barrel, and will reduce the likelihood of jamming.
Guns with a higher rate of fire like support guns (M249, M60, etc.) should not use tightbore barrels. Due to the massive number of bb's that pass through the gun, the chance of one jamming is higher.
Barrel Materials
Brass & Aluminum
Stock gun barrels will most often be made from aluminum, and sometimes brass.
The metal tightbore barrels require regular lubrication and care to extend longevity and maintain performance.
Teflon-coated
Some manufacturers produce barrels with teflon coated over aluminum. The teflon is more durable, resistant to friction/wear and scratching. Though the teflon barrels are designed to take more abuse they still should be lubricated and cleaned regularly.
An AEG should not have an inner barrel diameter of less than 6.03mm, otherwise expect frequent jamming. However, bolt-action sniper rifles may be optimized with 6.01mm barrels. Some companies
will manufacture a barrel to the specific bore and length you specify but it is cheaper to get
ones that are pre-made. BB's are typically 6mm in diameter but one or two manufacturers produce
rifles which fire 8mm bb's. Assume 6mm if size is not stated.
6mm bb's are really about 5.98mm in diameter +/- given the tolerances from different manufacturers.
Barrel Lengths
Upgrade barrels are made in various lengths. Before purchasing a tightbore, ensure that the barrel length matches the barrel being replaced. Longer barrels may replace shorter ones but the barrel needs external support and protection to maintain accuracy and last in the field. Barrel extensions or suppressors are common ways of lengthening the outer barrel.
Typical inner barrel lengths for rifles
110mm MP5-K
229mm MP5-A, MP5-SD
247mm G-36C, P-90, CAR-15
285mm MC51
300mm
363mm M4A1
407mm SG551
455mm AK-47, AK-47S
469mm G3
500mm M14
509mm M16A1, M16A2, AUG
550mm
590mm
650mm
Some brand comparisons
name
inner barrel diameter
material
JB Unicorn (JBU)
6.03mm
teflon-coated aluminum
KM/TN
6.04mm
teflon-coated aluminum
Mad Bull Black Python
6.03mm
T-6 7075 Aircraft Aluminum
Prometheus
6.03mm
aluminum
Note: brand used to produce 6.02mm inner barrels
STAR
6.05mm
brass
Systema
6.04mm
brass
Tokyo Marui (stock)
6.10mm
aluminum
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