Airsoft BB's - which ammunition is best for my replica?


I'm sure that everyone had the same dilemma „What bb’s should I choose to my airsoft replica”. Most of the time we choose bb’s to the fps that our replica has, but is it the best way? Today I would like to show you how I choose bb’s for my airsoft replicas. Before we start I would like to introduce you to the terminology that we use.

FPS (feet per second) – unit of measurement for muzzle velocity,

Joule J – unit of kinetic energy measure, this value translates into how far the bb will fly, and how much it hurts when you get hit.

When choosing airsoft bb’s I take in consideration a few things:

  1. Cylinder to barrel capacity ratio
  2. The environment I will play in (forest/CQB)
  3. BB quality / budget

Cylinder to barrel capacity ratio

For me, the first and most important factor for choosing my ammo is to determine the cylinder to barrel capacity ratio. Every cylinder type has a different capacity the same goes for barrels, longer the barrel more capacity it has, more air in the cylinder in ratio to the barrel, the heavier bb’s can be used. The most optimal ratio is between 1,80-3,20. It allows us to select the most common bb weight.

For example, if we want to use a 0,28g bb’s, the ratio should be about 2,5, for heavier 0,36g bb's, it should be about 3,20, as stated in the description under the table. On the internet, you can easily find calculators and tutorials on how to calculate the ratio. I use Szogun's table that I found a long time ago at (Original file is in Polish, this table is only translated screenshot).



Source: WMASG

As an example, I will use my Specna Arms SA-E10, which has a type 3 cylinder, and 170mm barrel. If we have a standard barrel length we can just check it in the chart. I have a non-standard 170mm, so I had to add it to the table. If we look at the cylinder to barrel ratio, we will see value 2,98 what, according to description, means that I have enough air to use even 0,32g+ bb’s. Let's check it out, but first, a control shot on 0,20g.

 chrono test of BBs


As you can see, replica generates 331,2 FPS and 1,01J. Now, let's try heavier bb’s.

chrono test of BBs


With 0,36g bb's we can observe a Joule increase from 1,01J to 1,16J. It means that the excess amount of air we had in the system was used to push a heavier bb. That effect is called Joule Creep and in this case, it means a free „power upgrade”. This confirms the result from the table - we can use heavy bb’s in the replica. When we will come back to value with 0,20g bb’s, we see that my replica generates about 353.8 FPS. All o that thanks to heavier bb’s. Additionally, it exceeds limits for CQB games.

chrono test


It’s not a huge problem because all I need to do is use some lighter 0,28g bb’s to drop the power to 1,1J. Calculated back to 0,20g bb’s gives me 345,1 FPS, I'm back in the limits.

This method shows us what range of bb weight we can choose from. It is best to select the weight of the bb’s to the amount of air in the system to prevent its excess, which can cause unstable bb flight, bigger spread and sometimes unwanted Joule Creep effect that can cause your gun to be above limits. Before we move next I would like to show you one more example.

 BBs chrono test

Specna Arms SA-B12 with a type 0 cylinder and a 455mm barrel. If we look into the table, we will see that the ratio value is only 2,26. It means that with bb’s heavier than ~0,25g I will start to lose power. With 0,20g bb’s I have 462,5 FPS/1,98J,

 chronograph test

but with 0,28g I see a small drop to 1,97J.

 chrono test of BBs


However, it gets worse when I use heavier bb’s - dropping to 1,91J. That’s why, to use heavier bb’s with this airsoft replica, I will need a shorter barrel.


I have determined what bb weight I can use in my replica. Now, it's time to think where I will use it.

Forest – on longer distances, that are characteristic for big sites and forest gameplays, it’s important for bb's to have straight flight not affected by wind, and has enough force to go through leaves and small twigs. That stability and force can be achieved by using a heavier bb’s from 0,28g+ range. Very popular (in Poland) are weights 0,30g, 0,32g and if your hop-up can spin it, even a 0,36g (I'm talking about AEG). Heavier bb’s will be less affected by wind and vegetation. It’s important to have in mind that a heavier bb flies slower than a light one, especially when we want to hit a fast-moving target.

CQB – in close quarters, where the action is fast and dynamic my priority will be speed, faster the bb, easier it will be to hit a fast-moving target without overcompensating for its path. If we look back at chrono results for SA-E10 we will see that the speed difference between 0,20g and 0,28g is ~40 FPS, we lose even more with 0,36g coming to ~70 FPS, that in CQB can make a huge difference. That’s why for close quarters I would recommend lighter bb’s in a range from 0,20g to max 0,28g, so our shots can get faster to the target. On indoor fields, the benefit from heavier bb’s is not relevant because there is no wind and vegetation.

Mixed fields - when we have a field that combines both outdoor and indoor gameplay, and we only have one airsoft replica, we will have to go for a compromise. With both the SA-E08 and SA-E10 replicas when I play on a mixed field I use 0,28g bb’s. They give me enough stability on longer shots, are good in CQB and I don’t have to worry about being over field limits.

BB quality / budget 

Knowing what bb weight we can use, and where we will use them, it’s only a question of choosing the right manufacturer. My advice is simple, High-quality bb’s is the cheapest upgrade.

Having a replica with a good barrel, nice bucking and with great internals we can choose the best bb weight that will use the Joule Creep effect to the max, and still, our shots will miss. Bb’s will fly to the right or the left. Hop-up will hop bb’s unevenly and they will jam in the mags, or worse, in the barrel. It sounds like a horror scenario, but it’s just a reality of badly selected bb’s dictated only by their price. If we want to use our replicas' full potential, we must use high-quality ammo. By my experience, I would recommend bb’s from BLS and G&G as top tier manufacturers. I'm sure you will not be disappointed. I'm aware that a beginner that counts every cent will choose some cheaper options. We don’t have to take the most expensive bb’s if we use a fully stock rifle. However, remember that we can have better results with better ammo. From time to time, I use Rockets Platinium or black Specna Arms in my Cyma Glock cm.030. I believe that in the case of stock replica, used at the distance of 5-20m, there is no need to invest in better bb’s.


I hope that the method I showed you, will help you to choose the right weight bb’s without using a chrono. I used it long before buying mine. Now, when I have it I can confirm it. Of course, as a perfect choice, it is to use a chronograph. It will show us how many Joules we get on different ammo. Selecting the right weight is not only determined by the power of our replica but also where we will use it. Doesn't matter if it’s a forest or CQB. I'm sure there is the right weight that will do the job. We need to remember that we will get the best results with high-quality bb's, but only if the replica is properly prepared. It's because, even if we use a high octane gas in Fiat 126P it won't change into a Porsche, despite having an engine in the back ;)




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