Tech Briefs

Data gathered from physical testing will be compared to analytical methods used to assess barrel wall temperature profiles and barrel wall stress and strain.

Currently in the small arms community, with the push for lighter, stronger barrels with improved life, a more complete understanding of the bore's thermal and structural behavior is required in order to not only improve future barrel design but to more thoroughly and accurately assess barrels in the current inventory.

The environment within a small caliber gun barrel during a firing event is extreme. As a round is fired, hot, pressurized gases are produced, ranging in the thousands of degrees Fahrenheit (°F) and up to and beyond 62,500 lb per square inch (psi), all within 1 ms. Moreover, in an automatic weapon such as a machine gun, the barrel's steady state temperature quickly increases as more rounds are fired in succession with the outer surface of machine gun barrels reaching higher than 1,000°F.

M240L long barrel

While the temperature at the barrel's outside wall can be easily measured in real time by a variety of means, the temperature at the bore or throughout the barrel's wall is much more difficult to characterize experimentally. Previous research aimed to characterize the true temperature experienced by a 5.56-mm test barrel's bore and throughout the barrel's wall in real time. Other research also looked at measuring the temperature through a 5.56-mm M4 barrel's wall and correlating the data back to numerical computational fluid dynamics (CFD) modeling and simulation tools. Data from those studies has been correlated back to various modeling and simulation of the respective 5.56-mm barrels; however, there is no information available on 7.62-mm barrels, such as the M240L long barrel. Data and related modeling and simulation from a 7.62-mm barrel, when combined with the data and modeling and simulation from the 5.56-mm barrels, will help develop a better understanding of barrel heating in small caliber barrels and could help to increase the fidelity of the modeling and simulation tools used to predict barrel temperatures.

Stress analysis of small caliber barrels is currently done using calculations in the Army Materiel Command Pamphlet (AMCP), Engineering Design Handbook, Guns Series, Gun Tubes, AMCP 706-252. There has been minimal work done to correlate test data back to the calculations outlined in AMCP 706-252. While strain is easily measured, traditional strain gages are limited in their utility on a machine gun barrel due to the temperature that the outside of the barrel reaches during firing. Correlation of the strain measured in the hoop direction to the strain calculated using traditional methods from AMCP 706-252 is critical to gaining a more thorough understanding of the barrel's structural behavior.

The purpose of this research is to characterize the temperature and stress in a 7.62-mm M240L long machine gun barrel during a firing event using commercial sensors and data acquisition equipment. The data from testing is intended to be used to correlate and validate numerical CFD modeling and simulation to predict barrel temperature, as well as gun barrel stress analysis outlined in AMCP 706-252. Temperature data was measured with in-wall thermocouples (IWTC) from Veritay Technology, Inc. (East Amherst, NY), and strain data was measured with strain gages from Vishay Precision Group (VPG), Inc. (Malvern, PA). The measurements were taken in real time and at high sampling rates so that the true profile during the firing event could be observed.

This work was done by Adam M. Jacob, Adam L. Foltz, and Laurie Florio for the Army Armament Research, Development and Engineering Center. ARDEC-0004

This Brief includes a Technical Support Package (TSP).

Characterization of Bore Temperatures and Stresses in Small Caliber Gun Barrels (reference ARDEC-0004) is currently available for download from the TSP library.

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