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Bite The Bullet

Is Tighter Better? (2008-07-18)

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FUNDAMENTALS OF ACCURACY

While many airsoft guns are fairly accurate out of the box already, there are always more demanding players out there with higher expectations. Having an upgraded AEG is part of the psychological grip you want over your opponents, but it's also more satisfying to shoot any weapon when the point of impact is where you intended.

The first thing to do before even touching the internals of the weapon is to choose good BBs and learn to shoot. Even a laser-like tackdriver won't be useful in the field if you shoot junk BBs through it, or if you never fired some shots at targets. Remember, practice for an airsoft game doesn't have to be boring bullseye-plinking from one position. Set up a practical range where you can and shoot big enough targets with your mind on the speed as well! However, that is not the subject of this test.

TEST CONDITIONS

The test was conducted in our office at 23C and a relative humidity of 57%. Our test weapon was a bog standard Tokyo Marui M733 Commando, and the BBs were chosen for precision and weight: Tokyo Marui 0.3g Superior Grade. A heavy precision made BB is less prone to inconsistencies, so we figured it would reduce the error margin compared to light bulk BBs. The compression was checked after each barrel swap, and 20 BBs were shot between test strings to bed in the hop-up and changes in setting.

Test strings were shot both with the Hop-Up completely off, as well as hop-up turned on enough to make the trajectory of the 0.3g BB straight. Normally Hop-Up has very little effect on muzzle velocity, because the amount of pressure to achieve a good spin for the light and fast 0.2g BB is small. With a heavier BB you need to crank it up because of the higher mass and lower velocity, and even minute changes at higher settings can affect the velocity quite a bit. The results with Hop-Up on are for reference, and do not necessarily reflect differences between any given barrel.

THE THEORY

The theory of improved velocity with a tightbore barrel is quite simple: Because the barrel bore (inner diameter) is smaller, there is a smaller gap between the BB and barrel, which means that less air is wasted blown by the BB. Also as the volume of the barrel is reduced, it is filled more quickly (=pushing the BB out more quickly) by the fixed amount of air displaced by the piston from the cylinder.

The nature of this kind of an improvement is dynamic. In layman's terms it means that the higher velocity you are pushing, the more difference a tightbore will make. Since we used a Tokyo Marui AEG as the standard, the difference is relatively small at this level. With an upgraded skirmish weapon at 450 fps the difference would most likely be bigger, just like aerodynamics matter more in an F1 racing car than your mom's "shopping bag on wheels".

How the Airsoft gun propels the BB has also an effect on the outcome. Gas pistols benefit from tighter barrels, because they are able to shoot the BB as hard as an AEG from such a short inner barrel. This means that the pressure behind the BB is high, and the reduced blow-by will transfer into velocity more efficiently. For example we've seen a Tokyo Marui 5.1 jump 30 fps higher just by adding a 6.01mm inner barrel! Gas guns are also known to benefit greatly from increased barrel length.

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THE TEST

First up was the original 300mm Tokyo Marui 6.08mm inner barrel. This represents the basic level, and any reduction below this level would not be pleasing from an upgrade part. The consistency was found to be very good already, and the high grade BBs are showing their effect. The velocity figure in brackets with 0.2 grams is an estimate, calculated like this: Energy (in joules) = 1/2 mass * velocity^2. A handy article and online calculator can be found here.

Average velocity with Hop-Up off: 235.8 fps, 0.77J (287.8 fps w. 0.2g)
Average velocity with Hop-Up on: 227.8 fps


Then we moved one step down in nominal size and pulled out the PDI 6.05mm barrel. PDI makes these 303mm long for the same reason dogs lick themselves: Because they can. The original 300mm inner barrel is just a little bit short actually, so it's good to see PDI making decisions of their own instead of blindly following how the original part was made.

Average velocity with Hop-Up off: 240.8 fps, 0.8J (293.4 fps w. 0.2g)
Average velocity with Hop-Up on: 228.4 fps


Next size down is Prometheus with their 6.03mm inner barrel. Prometheus became the mainstream tuning barrel over KM Head and Systema a few years ago, after rumours of the tightness causing jams was debunked. While precision barrels are more sensitive to dirt especially in terms of accuracy, an actual jam requires a badly enough deformed BB or fouled barrel that it would make pretty much any barrel jam. Prometheus also fared very well in a recent German test, especially in terms of roundness and true diameter. They are popular for a reason.

Average velocity with Hop-Up off: 238.4 fps, 0.79J (291.5 fps w. 0.2g)
Average velocity with Hop-Up on: 224.0 fps


Finally the tightest barrel is again made by PDI, this time in 6.01mm size. All PDI inner barrels and cylinders are cold-hammer forged, which is the same method as used to manufacture real sniper rifle barrels. The barrel is hammered from four sides by a hydraulic machine, while a round form is kept inside to produce the shape: Rifled for real rifles, round for PDI. This machinery costs more than a decent apartment and nice car in most parts of the world, and in our opinion the results are the finest in the market.

Average velocity with Hop-Up off: 247.0 fps, 0.85J (302.4 fps w. 0.2g)
Average velocity with Hop-Up on: 235.4 fps


CONCLUSIONS

What we witnessed here was pretty much as expected: A standard 6.08 barrel gave the lowest velocity, while the tightest 6.01 yielded the highest results, pushing the standard Tokyo Marui M733 Commando from 287 to 302 fps. The middle-ground is held by 6.03-6.05mm inner barrels, and their difference was neglible - although surprisingly for the benefit of 6.05mm one. While the primary reason for precision barrels is to gain accuracy, they also make your Airsoft gun more efficient as an often desired side-effect. It is important to notice that the results would vary with the power level as well as power source of the weapon, and possibly barrel length as well. It's a huge uncharted area, but here's a first step to prove that the talk about precision barrels isn't old wive's tales!

Thanks for reading!

Original velocity table for raw data:

Untitled Document
Hop-up off Hop-up on
Tokyo Marui 6.08mm 235 227
236 228
236 229
237 228
235 227
PDI 05 6.05mm 241 228
241 228
242 227
240 230
240 229
Prometheus 6.03mm 238 223
240 223
238 225
237 224
239 225
PDI 01 6.01mm 247 236
248 234
247 236
247 235
246 236