2010-03-26

Machine gun madness. Madness? This is ARMA!+blog

Ever spend time firing round after round at some poor bastard at range with a machine gun and while you watch the bullets hit all around but magically never touch him think "those stupid BIS/ACE devs gave too much dispersion to these guns"?

I know I have.

In fact, I was so convinced that if I could some how find dispersion specifications that the Army set to these MGs, I would have a strong case for recommending that they be decreased. So, Googling with the fervor of a pre-teen looking for dirty pictures, I found what I was looking for.

It turns out that the army has a very simple test to qualify their M240 and M249s for bullet dispersion. Using a hardpoint mount, they fire a burst of 10 rounds at a target 50m away. They discard the least favorable impact and require that 9/10 rounds land in a 33cm x 33cm area, with the mean of the rounds landing within a 20cm x 20cm square.

How to do that in game? Well luckily I had made THIS target previously. I was able to determine rough measurements using O2's measuring tool. I decided that for an ACE gun to pass the dispersion test, 9/10 of the rounds must hit within the square with most of the rounds landing within the white circle. (I know, I know - its not precice).

Methods man!

Testing was conducted by attempting 2-3 trials of firing 10 round bursts at this target from 50m, with bipod deployed to minimize recoil. To determine pass or failure, the sizes of the screen captures were normalized with the measurements picture in photoshop and overlayed to determined if 9/10 rounds landed within the allowable area. The negative control was the D10 HK416.

Results

Representative results are given below:

Conclusions

After analysis, the D10HK416 failed the dispersion test, since multiple rounds landed outside of the boxed area. The M249 and M240 machine guns performed the best, with the tightest groupings and most rounds landing within the white circle. Given its substantially shorter barrel (14.5"), the M249 Para passed the test but displayed a larger dispersion compared to the M249 and its 20" barrel. The MK48 consistently failed the criteria for dispersion that is expected of the M240 and M249.

Discussion

The results were generally surprising. The initial assertion that dispersion values for the M249 was too large has been proven unfounded. The M249, M249 para, and M240 machine guns all pass the US Army's test for dispersion as conducted at 50m. It is notable that down range, the observed bullet dispersion at 500m, for example is much greater from the M249 when compared to the M240. This is probably more closely related to recoil and ballistics values as modeled by the ArmA engine. As expected, the D10HK failed the testing due to its much shorter barrel and lack of bipod. Most surprising were the MK48 results. The dispersion with bipod deployed at 50m was great enough to cause failure of both the M240 and M249 specifications. While the observed bullet dispersion in-game would likely result in a real-world failure, dispersion specifications for the MK48 could not be located. Even so, the M240 and the MK48 both use an 18.8" barrel and should, therefore, have the same dispersion properties when recoil is a non-factor. Investigation of the weapon cfg revealed the following:

Dispersion Cfg value

M240                  0.003
MK48    0.004

Recoil:
ACE_M240_RecoilProne[] =    {0,0.01,0.0012, 0.09,0.009,-0.001, 0.15,0,0};
ACE_Mk48_RecoilProne[] =    {0,0.01,0.003, 0.09,0.009,-0.001, 0.18,0,0};

Not only does the MK48 have higher recoil, it also has higher dispersion. Apparently the combination of the 0.001 difference in dispersion and the increased recoil compared to the M240 causes the MK48 to fail the functional test and perform more analogous to the D10HK negative control. Reducing the MK48 dispersion to be equivalent to the the M240 would be a welcome change.

Taking a step back now, the most apparent and supportable conclusion is as follows: Scubaman3D is clearly a nerd.