Joules, You Idiots!

by Zach Billings on March 23, 2011

Airsoft BB Mid AirMy biggest pet peeve in the airsoft world is the way that people talk about the power of a gun. The standard for measuring an airsoft gun’s power is to chronograph a 0.2g bb fired from the gun. This system is used throughout most of North America and the world for establishing field limits as well. A common limit for an outdoor airsoft field might be 400fps with 0.2g bbs. The field that I frequent has its limit set at 400fps (feet/second) with 0.25g bbs (0.25s being the less common standard for measurement). Why on Earth do we use velocity as a standard if bb weight is a variable?!

To give you a little background, this is how it works. I show up at the field with my airsoft rifle. I chronograph the gun using ammunition provided to me by the field, to ensure that no one cheats. I fire three rounds and so long as they all clock under 400fps with the supplied 0.25s, I pass. Then for the rest of the day I actually utilize 0.28 or 0.30g bbs. The additional weight of the bb decreases my velocity at the muzzle (the end of the barrel) but the energy carried by the bb remains a constant, at around 1.85J (Joules).

The point behind using a heavier bb is that they resist wind more. The heavier your bb, the less it’s effected by side to side wind and the more slowly it looses energy as it flies down-range.

People use a huge variety of bb weights in their guns. The light 0.2g bbs are usually reserved only for players who are not in-the-know. 0.25s are commonly used for pistols and less-powerful rifles. For players who understand bb flight characteristics and have a moderately powerful rifle, the weights used are 0.28, 0.30, 0.36, and 0.43 grams. 0.36 and 0.43 are usually [incorrectly] reserved for snipers.

Because every player actually plays will a different weight, it seems asinine to me that the standard for measurement is with a single bb weight at x velocity. Joules should be used as the standard, and velocity-with-weight would only be used for chronographing at the field.

The reason this came up is that I am a regular poster on Airsoft Retreat, and since I don’t use 0.2g bbs for any purpose, I don’t own any. Now, when I go to chronograph a gun at home to post information about it, I have to use results achieved with .25s. Most people don’t know how to convert these results and only understand velocity measurements given with 0.2g bbs. Lets inject a little science into the community and get a standardized system going!

Here’s some additional information on why I use heavier bbs (I will be using 0.36 or 0.43g this year).

Many players gripe about a heavier bb causing a lower muzzle velocity, but here’s the catch: Lets say I fire a 0.25 at 400fps. That’s about 1.85J. That means that a 0.43 fired from the same gun would leave the barrel at only 290fps. To most players this seems like a hinderance. Nay. Lets go 21m (69ft) down range. The 0.25 was greatly effected by the wind and has now slowed to 203fps. Meanwhile the 0.43, which started 110fps slower, is now traveling faster at 205fps. Most people with guns shooting above 1.6J are regularly engaging targets beyond 100ft. At those ranges, not only are the 0.43s now going faster, but they are carrying vastly more energy to continue their journey with. At the aforementioned 21 meters, the 0.25 has dropped from 1.85J at the muzzle to only 0.47J. The 0.43 has retained much more energy and is still carrying 0.84J of the original 1.85J.

There are tradeoffs involved which don’t always make heavier better. If you’re shooting at ranges beyond 50 meters then heavier is always better, because at 50m the average speed of all bb weights is about the same. If, however, you are engaging in urban combat at close ranges, you may want to stick to something like a .28. At a mere 12 meters, the .28 will reach its target more quickly and at that range you don’t need to be as concerned with conserving stored energy. You just want it to get there as immediately as possible.

This year I plan to carry a combination of 0.28s for close quarters use, and 0.36s for long range use.

{ 4 comments… read them below or add one }

Alex February 6, 2013 at 11:42 am

My AEG shoots about 454 fps with the hop up correctly set for the weight of 0.20 grams bb. (almost compleetly turned off or the bbs curves up). This would convert to aprox 1.9 joules

I use a 560 mm 6.02 tightbore barrel, and I have experianced that it performes the most accurate with 0.40 gram bbs.

How ever when I shoot 0.40 gram bb I need to use more hopup.
I chronographed the 0.40 bbs with the hopup set correctly, and It shoot only 265 fps. I realise that the results would be different with the hopup turned compleetely of, but in the field, this would never be the case.

Now in theory the 0.40 gram bbs should be shooting at 321 fps which would convert to 1.9 joules.

When mesuring the 0.40 gram bb I got a reading of 265 fps which is a joule drop to 1.3

I was expecting a little drop in joules because of more resistance in the hopup, but not a drop as big as 0.6

I wonder if a heavier bb perhaps needs a lot more energy to start accelerating from zero fps then a lighter one, and that only after it starts to move, the energy output is getting the same effect?

If this is the case, then it would mean that you can upgrade your gun more, as long as you only use 0.40 gram bbs, get more accuracy and range, without hitting your targets with more energy because the joules would be not be changed…

Please give me some good thoughts about this….

Zach Billings February 6, 2013 at 1:35 pm

Try some different heavy-weight bbs. There are always variables that are harder to accurately calculate. BB diameter is one. The BBs you’re using could be .01-.04mm lesser in diameter, compared to your .20g control. If this is the case you would be allowing more of the compressed air to pass around the BB. This would only reduce expected velocity by about 2-7% but that’s one factor. Then there’s the hop-up. With the hop-up cranked up you lose about 5% compared to the hop-up barely engaged. Now you’re talking a total loss of around 5%.

It is also true that there are some factors outside my understanding in regards to what effect a heavier BB has. It is certainly possible that it takes more energy to accelerate and therefore some is lost to this effect. More likely this is due to the fact that velocity is the exponential factor in the equation for energy. For this reason when you use a higher weight BB you lose velocity, proportional to mass. This means you lose energy. The same effect can be seen in real steel bullet ballistics. Using the same powder charge, a heavier bullet will always have less energy. In airsoft, you are trading raw energy and speed for increased accuracy and consistency.

Your finding are not that surprising, especially with the hop-up factor taken into account.

Recommendations…

Get a shorter barrel. 509 is the standard sized barrel for airsoft M16s. I really wouldn’t go past that. It is a myth that you get increased accuracy at that point. In reality you just risk adding drag to the BB through loss of barrel pressure. I attained an effective range of 280ft (could consistently hit a torso in fair weather) with a KWA M4A1 AEG with a 363mm 6.03mm barrel. It’s not about the barrel length. Get a KWA 2G hop-up bucking and make sure you’re using good ammo and you’re set.

I don’t tend to recommend higher spring power or anything like that to get more velocity or energy. Get a new cylinder head o-ring to increase cylinder pressure. You should be able to hold your finder over the air nozzle and push the piston hard through the cylinder. The cylinder should pressurize and you should not be able to push the piston all the way through.

Hope this helps,
Zach

Alex February 6, 2013 at 2:46 pm

Thanks for your fast reply Zach!

I just bought my 560 mm PDI 6.01 barrel (though it is messured to be 6.02 wide) and I can tell you it was expensive, so I am hoping that it will do a good job.

I saw this German test of tightbore barrels and PDI came out as an exelent manufactor: http://www.madbullairsoft.com/images/Advertising/MadBull%20Barrel%20Report.pdf

I have upgradet both the cylinder, air nozzle and o-ring, but perhaps they are wearing down over time?

Anyways my rifle is now “headshot accurrate” at a distanse of 140 feet with 0.40 gram bbs, but looses it range from that point.
If i turn up the hopup, the barrel gets too tight, so my plan was to increase the fps with the 0.40.

My only fear is that it then becomes dangerous enough to risk penetrating skin and so on.

Alex February 7, 2013 at 7:28 am

I have been using King Arms 0.40 and Devil Blaster 0.40 bb’s btw.
They are supposed to be high quality seamless bb’s with little or no variation in size.

(Sorry for my bad english btw, I am from Scandinavia)

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