.20g vs. .25g
Article written by Jared, Mechanical Engineer and Airsoft Player
published March 2005
So you finally bought
your first AEG, gas, or bolt airsoft gun and the next thing on your
mind is shooting it. Before you can do that, you’ll need some
bbs, but which weight should you buy? There are numerous weights of
bbs out there available for purchase starting with weights as low as
.12 gram all the way up to .88 gram carbon steel bbs. The majority
of the bbs sold today are .20 gram. As you go up in weight, you will
go up in price and down in quantity of your bag of ammo. But who
cares about the weight of the ammo you buy? Most people want the
most for their money, so why not go with the cheapest .20 gram bbs
you can find?
There have been
numerous field studies performed that deal with specific bb weights
related to accuracy, fps, and range. Since these are the three main
areas of study for airsoft ballistics, we should look at them more in
depth than just recording data trends from the field. Although this
method of using statistical data to form a trend best represents
actual effects in these three areas, is it possible to apply science
and physics behind these findings to further knowledge in these
specific areas?
The answer is yes. The
study outlined in the following paragraphs shows the effects of three
variables on airsoft ammo. These three variables are time, fps, and
distance. The main reason for this study was to find downrange
velocities of certain bb weights at varying distances and specific
time intervals. This particular study uses a theoretical airsoft gun
firing at 85m/s (278.87 fps) and compares the relations between .20
and .25 gram ammo. It can be altered to any muzzle fps as well as
any ammo to show the desired results on the tables and graphs from
only two inputs (bb weight and initial m/s). This study not only
includes the effect of specific bb weights and initial muzzle
velocities but also uses an exponentially changing drag coefficient
and corresponding interval velocity to account for the drag forces
that act on the bb during flight. Without these drag forces, the bbs
velocity at every distance would be constant. We know this is now
true because it hurts much worse to be hit by a bb from 10 feet
versus 100 feet away.
The spreadsheet below
has many columns of numbers and formulas that are important to the
study, but not directly relevant to understanding the final result.
They are all in SI units, not English, so do not be confused. The
final results are in English units since most of us prefer and
understand a measure in feet versus meters. This first sheet shows a
.20 gram bb fired at a theoretical stock fps of 85 m/s (278.87 fps).
For those interested in details, the variables from left to right are
mass of bb in grams, initial velocity in m/s, time, total distance
traveled in meters, coefficient of drag, area of bb perpendicular to
velocity flow, the density of air at STP (standard temperature and
pressure), force of drag in Newtons, acceleration in meters per
second^2, velocity in fps, time, and total distance traveled in feet.
|
INPUT
|
|
OUTPUT
|
|
|
|
|
|
|
m
(gram)
|
v
(t) (m/s)
|
t
(sec)
|
x(t)
(m)
|
Fd
(drag force N)
|
a
(m/s^2)
|
v
(t) (fps)
|
t
(sec)
|
x(t)
(ft)
|
|
0.20
|
85.00
|
0.0
|
0.00
|
0.03062363
|
153.118
|
278.87
|
0.0
|
0.00
|
|
0.20
|
69.69
|
0.1
|
6.97
|
0.02058435
|
102.922
|
228.63
|
0.1
|
22.86
|
|
0.20
|
59.40
|
0.2
|
12.91
|
0.01495317
|
74.766
|
194.87
|
0.2
|
42.35
|
|
0.20
|
51.92
|
0.3
|
18.10
|
0.01142559
|
57.128
|
170.34
|
0.3
|
59.38
|
|
0.20
|
46.21
|
0.4
|
22.72
|
0.00904956
|
45.248
|
151.59
|
0.4
|
74.54
|
|
0.20
|
41.68
|
0.5
|
26.89
|
0.00736398
|
36.820
|
136.75
|
0.5
|
88.22
|
|
0.20
|
38.00
|
0.6
|
30.69
|
0.00612044
|
30.602
|
124.67
|
0.6
|
100.69
|
|
0.20
|
34.94
|
0.7
|
34.18
|
0.00517435
|
25.872
|
114.63
|
0.7
|
112.15
|
|
0.20
|
32.35
|
0.8
|
37.42
|
0.00443643
|
22.182
|
106.14
|
0.8
|
122.76
|
|
0.20
|
30.13
|
0.9
|
40.43
|
0.00384893
|
19.245
|
98.86
|
0.9
|
132.65
|
|
0.20
|
28.21
|
1.0
|
43.25
|
0.00337302
|
16.865
|
92.55
|
1.0
|
141.90
|
|
0.20
|
26.52
|
1.1
|
45.91
|
0.00298176
|
14.909
|
87.02
|
1.1
|
150.61
|
|
0.20
|
25.03
|
1.2
|
48.41
|
0.00265597
|
13.280
|
82.13
|
1.2
|
158.82
|
|
0.20
|
23.70
|
1.3
|
50.78
|
0.00238165
|
11.908
|
77.77
|
1.3
|
166.60
|
|
0.20
|
22.51
|
1.4
|
53.03
|
0.00214837
|
10.742
|
73.86
|
1.4
|
173.98
|
|
0.20
|
21.44
|
1.5
|
55.17
|
0.00194825
|
9.741
|
70.34
|
1.5
|
181.02
|
|
0.20
|
20.47
|
1.6
|
57.22
|
0.00177523
|
8.876
|
67.14
|
1.6
|
187.73
|
|
0.20
|
19.58
|
1.7
|
59.18
|
0.00162458
|
8.123
|
64.23
|
1.7
|
194.15
|
|
0.20
|
18.77
|
1.8
|
61.05
|
0.00149257
|
7.463
|
61.57
|
1.8
|
200.31
|
|
0.20
|
18.02
|
1.9
|
62.86
|
0.00137621
|
6.881
|
59.12
|
1.9
|
206.22
|
|
0.20
|
17.33
|
2.0
|
64.59
|
0.00127311
|
6.366
|
56.86
|
2.0
|
211.91
|
|
0.20
|
16.69
|
2.1
|
66.26
|
0.00118131
|
5.907
|
54.77
|
2.1
|
217.38
|
|
0.20
|
16.10
|
2.2
|
67.87
|
0.00109919
|
5.496
|
52.83
|
2.2
|
222.67
|
|
0.20
|
15.55
|
2.3
|
69.43
|
0.00102545
|
5.127
|
51.03
|
2.3
|
227.77
|
|
0.20
|
15.04
|
2.4
|
70.93
|
0.00095896
|
4.795
|
49.35
|
2.4
|
232.71
|
|
0.20
|
14.56
|
2.5
|
72.39
|
0.00089879
|
4.494
|
47.77
|
2.5
|
237.48
|
|
0.20
|
14.11
|
2.6
|
73.80
|
0.00084417
|
4.221
|
46.30
|
2.6
|
242.11
|
|
0.20
|
13.69
|
2.7
|
75.17
|
0.00079443
|
3.972
|
44.92
|
2.7
|
246.60
|
|
0.20
|
13.29
|
2.8
|
76.50
|
0.00074900
|
3.745
|
43.61
|
2.8
|
250.97
|
|
0.20
|
12.92
|
2.9
|
77.79
|
0.00070739
|
3.537
|
42.38
|
2.9
|
255.20
|
|
0.20
|
12.57
|
3.0
|
79.04
|
0.00066919
|
3.346
|
41.22
|
3.0
|
259.33
|
*The chart assumes 3 constants: Drag=0.47, BB Area= 0.00001395 and Air Density=1.293
This sheet shows the
effects of a .20 gram bb fired at 85 m/s (278.87 fps) over a three
second time period with 0.1-second time measurement intervals.
Although we know that a bb fired from a stock gun at an almost
horizontal shot will not stay in the air for 3 seconds, it is
important to show the trend involved with this length of time. This
length of time will not affect our final results because they are
located all within a 1 second time frame. The last three columns are
what we are interested in. They show, from left to right, the bb
velocity in fps, the time corresponding to the velocity, and the
distance in feet at the same time.
The same variables are
used in the sheet for .25 gram bbs. Please note that the same
theoretical airsoft gun is used, so the initial velocity is reduced
(from 85 m/s to 76.02 m/s) to account for the heavier bb that retains
the same amount of joules of energy. If this is not easy to
understand, Covert of Canton has the formula for joule energy and a
link to an online joule calculator on their website that can be found
here
http://dan.mahonstudios.com/covertofcanton/insidepages/about.cfm
|
m
(gram)
|
v
(t) (m/s)
|
t
(sec)
|
x(t)
(m)
|
Fd
(drag force N)
|
a
(m/s^2)
|
v
(t) (fps)
|
t
(sec)
|
x(t)
(ft)
|
|
0.25
|
76.03
|
0.0
|
0.00
|
0.02449870
|
97.995
|
249.43
|
0.0
|
0.00
|
|
0.25
|
66.23
|
0.1
|
6.62
|
0.01859014
|
74.361
|
217.28
|
0.1
|
21.73
|
|
0.25
|
58.79
|
0.2
|
12.50
|
0.01464983
|
58.599
|
192.88
|
0.2
|
41.02
|
|
0.25
|
52.93
|
0.3
|
17.79
|
0.01187494
|
47.500
|
173.65
|
0.3
|
58.38
|
|
0.25
|
48.18
|
0.4
|
22.61
|
0.00983926
|
39.357
|
158.07
|
0.4
|
74.19
|
|
0.25
|
44.24
|
0.5
|
27.04
|
0.00829744
|
33.190
|
145.16
|
0.5
|
88.70
|
|
0.25
|
40.93
|
0.6
|
31.13
|
0.00709929
|
28.397
|
134.27
|
0.6
|
102.13
|
|
0.25
|
38.09
|
0.7
|
34.94
|
0.00614827
|
24.593
|
124.95
|
0.7
|
114.63
|
|
0.25
|
35.63
|
0.8
|
38.50
|
0.00537989
|
21.520
|
116.88
|
0.8
|
126.31
|
|
0.25
|
33.47
|
0.9
|
41.85
|
0.00474960
|
18.998
|
109.82
|
0.9
|
137.30
|
|
0.25
|
31.58
|
1.0
|
45.01
|
0.00422578
|
16.903
|
103.59
|
1.0
|
147.66
|
|
0.25
|
29.88
|
1.1
|
47.99
|
0.00378545
|
15.142
|
98.05
|
1.1
|
157.46
|
|
0.25
|
28.37
|
1.2
|
50.83
|
0.00341157
|
13.646
|
93.08
|
1.2
|
166.77
|
|
0.25
|
27.01
|
1.3
|
53.53
|
0.00309127
|
12.365
|
88.60
|
1.3
|
175.63
|
|
0.25
|
25.77
|
1.4
|
56.11
|
0.00281467
|
11.259
|
84.54
|
1.4
|
184.08
|
|
0.25
|
24.64
|
1.5
|
58.57
|
0.00257410
|
10.296
|
80.85
|
1.5
|
192.17
|
|
0.25
|
23.61
|
1.6
|
60.93
|
0.00236350
|
9.454
|
77.47
|
1.6
|
199.92
|
|
0.25
|
22.67
|
1.7
|
63.20
|
0.00217804
|
8.712
|
74.37
|
1.7
|
207.35
|
|
0.25
|
21.80
|
1.8
|
65.38
|
0.00201384
|
8.055
|
71.51
|
1.8
|
214.50
|
|
0.25
|
20.99
|
1.9
|
67.48
|
0.00186774
|
7.471
|
68.87
|
1.9
|
221.39
|
|
0.25
|
20.24
|
2.0
|
69.51
|
0.00173716
|
6.949
|
66.42
|
2.0
|
228.03
|
|
0.25
|
19.55
|
2.1
|
71.46
|
0.00161996
|
6.480
|
64.14
|
2.1
|
234.45
|
|
0.25
|
18.90
|
2.2
|
73.35
|
0.00151435
|
6.057
|
62.01
|
2.2
|
240.65
|
|
0.25
|
18.30
|
2.3
|
75.18
|
0.00141885
|
5.675
|
60.03
|
2.3
|
246.65
|
|
0.25
|
17.73
|
2.4
|
76.95
|
0.00133219
|
5.329
|
58.16
|
2.4
|
252.47
|
|
0.25
|
17.20
|
2.5
|
78.67
|
0.00125331
|
5.013
|
56.42
|
2.5
|
258.11
|
|
0.25
|
16.69
|
2.6
|
80.34
|
0.00118129
|
4.725
|
54.77
|
2.6
|
263.59
|
|
0.25
|
16.22
|
2.7
|
81.96
|
0.00111537
|
4.461
|
53.22
|
2.7
|
268.91
|
|
0.25
|
15.78
|
2.8
|
83.54
|
0.00105486
|
4.219
|
51.76
|
2.8
|
274.08
|
|
0.25
|
15.35
|
2.9
|
85.08
|
0.00099919
|
3.997
|
50.37
|
2.9
|
279.12
|
|
0.25
|
14.95
|
3.0
|
86.57
|
0.00094784
|
3.791
|
49.06
|
3.0
|
284.03
|
*The chart assumes 3 constants: Drag=0.47, BB Area= 0.00001395 and Air Density=1.293
Again, we are
interested in the last thee columns of data. Since all of this data
is hard to understand, this data is formed into three separate charts
that relate these three variables to each other graphically.
As you can see in the
first graph we have constructed, lines for the .20 and .25 gram bbs
are visible. This graph shows the relation of the data calculated in
the two previous spreadsheets. Specifically, it shows the relation
of velocity versus distance. The conclusion drawn from this graph
shows that .20 gram bbs have a much higher initial velocity and they
retain that velocity until they reach around 50 feet. This is the
point at which the .25 gram bbs retain their energy longer and
better, which results in a lengthier sustainable fps. Since the .20
gram bbs have a higher initial velocity, the drag force applied to
the bb is much higher. Since they are lighter than the .25 gram bbs,
they cannot retain their momentum as long; therefore, they lose
velocity at a more rapid pace than their heavier counterparts. Also,
the loss of momentum relates to the stability of the bb during flight
through the air, which in result is the reason why .25 gram bbs are
more accurate at longer distances.
The next graph shows
the relation of velocity versus time applied to the .20 and .25 gram
bbs:
This graph is
illustrates the velocity of each bb over specific time intervals.
Here, it is again obvious that the .20 gram bbs, since they are
lighter, have a higher initial velocity. Again, due to the drag
forces involved with higher velocities and the lower weight of the
.20 versus .25 gram bbs, they will lose their velocity and drop below
that of the .25 gram bbs after only 0.25 of a second. This is very
important to us in the area of specific shooting conditions. If you
are playing close quarter battles or assaulting, it might seem more
feasible to use .20 gram bbs rather than .25 gram. This is because
they travel faster to your target as long as the flight path from
muzzle to target will take less than 0.25 of a second. If you are
support or sniper based, this would recommend using .25 gram bbs
because they retain their velocity longer at lengthier flight time
intervals over 0.25 of a second.
The final graph shows
the relation of distance versus time:
This graph is important
to us because it shows the distances attainable with .20 gram versus
.25 gram bbs based on a time-oriented scale. It is somewhat hard to
see but for the first 0.5 of a second, the .20 gram bbs will reach
their target quicker. This will be the case from 0-90 feet as shown
on the graph as an intersection of the two lines. At this point, the
two bbs will both reach their target at the same time. After this
time, the .25 gram bbs will travel further quicker because they do
not lose their momentum as well as velocity as quick as the .20 gram
bbs. This shows that for shots less than 90 feet use .20 gram bbs,
for anything over, this study would recommend .25 gram bbs.
The results found in
this study do not include airsoft shots that are made far from
horizontal. This means that shots made up or down a large incline
would skew the results due to the earth’s acceleration due to
gravity affecting the acceleration due to drag. This could be
incorporated into the study but is very negligible for most airsoft
shots that are not shot at a steep initial angle. Also, hop-up is
not used here to determine range or drag because there is simply not
a formula in physics that simply states “for hop-up
calculations.” Hop-up will not effect the drag because drag is
only related to cross sectional area, which is unchanging during
flight since we have a solid, round bb. The effects of hop-up on
range are very noticeable, but here, we do not have an ultimate
range, solely intermediary values. Since both bb weight cases are
treated the same way, the results with hop-up would also yield
similar results.
Overall, the important
results to us are as follows:
- Shots made within 50
feet will have a greater ending velocity with .20 gram bbs.
- Shots made over 50 feet
should use .25 gram bbs for stability and velocity retention.
- BB flight times under
0.25 of a second can use .20 gram bbs for a higher velocity.
- BB flight times over
0.25 of a second should use .25 gram bbs because velocity and
momentum is conserved better.
- Shots of less than 90
feet can use .20 gram bbs for a faster shot time.
- Shots made over 90 feet
should use .25 gram bbs for a quicker, more stable ammo flight path.
The main reason for
this study was to find out if .25 gram bbs are actually better than
.20 gram bbs and at what velocities, times, and distances this is
true. All of this information might sound like another language to
some people, but this study was important in understanding the
question at hand and has supplied very useful information that is
easily comprehendible through simple graphical interpretation.
The previous
spreadsheets and graphical depictions are very useful when
determining the correct weight bbs to use with specific initial
velocities. If you would like a copy for personal use, please direct
your email request to the email listed below.
This information is
fully protected under the copyright of Covert of Canton ©
Copyright 2005 www.covertofcanton.tk. Any partial or full
reproduction of this article is strictly prohibited without full
consent of the author. Any reproduction requests, personal
spreadsheet requests, questions, comments, or concerns can be
directed to Zinger084 AT hotmail.com
|
