SURG_Attachment_04_Flash_Test_Report.pdf

Measurement of Suppressed MK18-MOD1 Muzzle Flash

Crane, Division

Naval Surface Warfare Center Crane, Indiana

47522 5001

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Commander, Crane Division, Naval Surface Warfare Center, Code JXNL, 300 Hwy 361, Crane IN 47522-5001. Distribution authorized to the U. S. Department of Defense, U.S.

DoD contractors, and Government Agencies of NATO and FVEY ONLY.

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Introduction:

The muzzle flash produced by small arms weapons fire is a combination of both visible and invisible (primarily infrared) light. A bright, visible flash is typically the result of sodium and/or potassium atomic emission produced during combustion of gasses after they leave the barrel.

Since this flash is the result of a secondary combustion event, it is commonly referred to as “secondary flash.” When functioning properly, a visible flash suppressor will typically prevent this combustion event, resulting in much lower levels of visible light emission. Under these conditions, the predominant visible emission is proposed to be from blackbody radiation hot gasses. Even though it is not as visible to the naked eye, suppressed muzzle flash still produces infrared radiation which can be readily measured. This emission is intrinsic to the hot gasses expelled from propulsion of the projectile, and is commonly referred to as “primary flash.”

Methods and instrumentation are being developed at the Naval Surface Warfare Center (NSWC) Crane Division to quantitatively measure the visible and infrared light produced during flash events. Though these methods are not currently mature enough to be considered definitive metrics of suppressor performance, measurement of the combined visible and near-infrared (NIR) light produced during a muzzle flash can currently provide a method for comparing the relative performance of various weapons within a given test session. To this end, the flash produced by three different MK18-MOD1 Carbines fitted with new FMBS-C suppressors were measured using an unfiltered silicon photodiode to provide a baseline measurement of the visible and NIR (~400 nm to 1050 nm) light produced by the muzzle flash from these weapons.

Test Method:

Three different MK18-MOD1 carbines fitted with new FMBS-C suppressors were fired in the NSWC Crane Small Arms Branch underground test range in order to quantitatively measure the muzzle flash produced. All lights in the tunnel were turned off during firing to reduce ambient light levels. Each weapon was fired from a seated position with the weapon resting on a bipod which was placed against a fixed stop to improve reproducibility of the firing position between each shot. After ten shots, the weapon was allowed to cool for at least 30 minutes before another set of ten shots was fired from the respective weapon. Three series of ten shots were measured, though the first set of results for each weapon were not reported since instrumentation settings were changed during those shots. The second and third sets of ten shots for each weapon are reported herein.

Instrumentation:

Light from the flash event was collected using a 2 inch diameter lens (150 mm focal length) and focused onto an unfiltered photodiode (model SED033, International Light, Inc.). The front surface of the collection lens was placed ten feet (10’) from the approximate center of the flash event and 5 degrees from the line of fire such that a head-on view of the flash event was measured. The photodiode current was passed through a transimpedance amplifier (TTI Inc.)

and the resulting voltage was recorded using a computerized analog to digital data acquisition system (National Instruments cDAQ with a collection rate of 100 kHz) using custom software written in LabVIEW. The gain and offset settings of the transimpedance amplifier were kept constant for all measurements and calibration. The recorded response was integrated over the first 20 ms of the flash event to calculate the radiant energy produced.

The response of the detector was calibrated against a 1000 °C blackbody source (1” cavity) fitted with a variable aperture wheel. The 50% response wavelengths for an unfiltered silicon photodiode were used to define spectral band limits of 525 nm to 1050 nm. The spectral radiance of this 1000° C source was calculated (assuming an ideal blackbody as described by Plank’s law) to be 257.11 W sr×m2 within this band. (Note: Definition of the high energy, or short wavelength, boundary of this band does not significantly impact the calibration value since the blackbody emits little energy in this region. Conversely, the sharp cutoff of the silicon photodiode provides a clearer definition of the low energy, long wavelength, and boundary of the band.) Five apertures ranging from 12.7 mm to 1.6mm were placed in front of the blackbody cavity to provide a three order of magnitude range of radiant intensity values for calibration. The silicon photodiode exhibited a linear response over this range. A linear fit of the measured voltages corresponding to these intensities was used to calculate a calibration factor. The blackbody aperture was placed six feet (6’) from the front of the collection lens, and the difference in distances between calibration and measurement conditions was accounted for during data processing.

Results and Discussion:

The first shot fired from the MK18-MOD1 (while at ambient, i.e., “cool” temperatures) typically produced a much larger radiant energy than subsequent shots, as is qualitatively shown in Figure

1. Since the intensity of this “cool” flash saturated the photodiode used for this test, the results of the first shot of each series were discarded.

Figure 1: Difference in intensity between a "cool" (left) and "warm" (right) suppressed muzzle flash.

A typical measured muzzle flash for a warmed MK18-MOD1 fitted with an FMBS-C suppressor is shown in Figure 2. The duration of a primary flash for the suppressed MK18-MOD1 was less than 2 ms, which suggests that the total light emitted during the event, or radiant energy, is a more meaningful metric than peak radiant intensity. The radiant energy can be easily calculated by integration of the transient radiant intensity over the duration of the primary flash.

Figure 2: Transient suppressed MK18-MOD1 muzzle flash radiant intensity

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Ra di an t I nt en si ty

(W /s r)

Time (ms)

Mk18 Suppressed Muzzle Flash Weapon 10004752, Series 2, Shot 7

The muzzle flash from each of the remaining nine shots within each series (Table 1) resulted in an average 0.214 mJ/sr measured radiant energy (standard deviation of 0.038). These results show that the flash intensity is generally consistent both within each test series and between different weapons. The flash energies do appear to drift higher as the weapon warms within each test series, and from the first to the second test series, though these differences are not large enough to suggest that the statistically significant based on the sample size. Variation of the flash intensity as a function of weapon temperature is beyond the scope of this study and report.

Though collected data does suggest that testing procedures be used for comparison of different weapon units should specify that a similar firing schedule should be used for each weapon to ensure that any thermal effects on muzzle flash are considered.

Table 1: Radiant energies produced by suppressed MK18-MOD1 muzzle flash.

Weapon ID 10004752 10004782 10004793 Series 1 Series 2 Series 1 Series 2 Series 1 Series 2 mJ/sr mJ/sr mJ/sr mJ/sr mJ/sr mJ/sr

Shot 2 0.136 0.226 0.233 0.215 0.180 0.212 Shot 3 0.153 0.214 0.170 0.186 0.191 0.201 Shot 4 0.138 0.231 0.172 0.211 0.180 0.223 Shot 5 0.144 0.228 0.193 0.210 0.172 0.207 Shot 6 0.158 0.209 0.179 0.223 0.182 0.230 Shot 7 0.165 0.251 0.215 0.238 0.201 0.222 Shot 8 0.194 0.255 0.238 0.257 0.231 0.211 Shot 9 0.212 0.264 0.237 0.285 0.235 0.232 Shot 10 0.227 0.271 0.276 0.286 0.249 0.290 Average 0.170 0.239 0.212 0.235 0.202 0.225

Stdev 0.033 0.022 0.036 0.035 0.029 0.026 Total Average 0.214

Stdev 0.038

It should be emphasized that the instrumentation used for this data collection has not been extensively tested, and the absolute values reported should be treated as preliminary values. The use of silicon photodiodes for radiometric measurements does, however, have a proven track record, and the methods used for this study have followed established methods. This method can provide a very good, repeatable “apples to apples” comparison in side-by-side testing—the absolute accuracy of the measured radiant energies has not, however, received sufficient scrutiny to establish a level of confidence. This instrumentation is currently undergoing refinement and validation, and should be able to provide absolute radiometric intensities when this process is complete.