The overall goals of this Environmental Security Technology Certification Program (ESTCP) project were to eliminate chromium coatings in Army gun tubes and to develop a technology that will extend their service life. The specific objectives of this demonstration were to confirm the results of previous firing tests and to obtain a better estimate of the extension to the service life that the explosively-bonded tantalum alloy liner provides. The demonstration used the M242 25-mm Bushmaster automatic cannon. However, the technology can be applied to other medium caliber gun tubes and possibly small caliber tubes.
The technology involved explosively bonding a refractory metal liner to the inside of the gun tube. Tantalum-10% tungsten (Ta-10W) had been chosen as the liner material. The procedure started with obtaining a steel barrel blank with an appropriately-sized through hole. The liner was inserted into the barrel blank, centered appropriately, and then filled with explosive. The explosive was detonated, driving the liner into the steel wall and effectively welding it to the gun tube. The procedures and materials used to machine the gun tube to final form were different from those used in manufacturing standard tubes and constitute an enabling technology. This was true whether more traditional techniques (crown broach cutting) or newer technology (waterjet cutting) were used.
Firing tests at Yuma Proving Ground confirmed and extended earlier tests. Previous tests had shown that the use of a Ta-10W liner can extend the service life of the M242 by a factor of three. These tests ended before the gun tube was worn out. The recent series of tests had shown that the Ta-10W liner can extend the life of the tube by a factor of four. Again, the tests ended before the gun tube was worn out. The high cost of testing due to numerous barrel inspections, low program priority leading to lengthy downtimes between test series, amount of ammunition needed, and extensive test instrumentation/data recording may preclude further attempts to determine the actual service life extension provided by the Ta-10W liner.
A major consideration in the implementation process is cost. An estimate puts the cost per barrel at $13,350, which is about twice that of a conventional one. The extension in service life by a factor of four using this technology argues for its adoption, even ignoring directives to eliminate chromium coatings. However, user acceptance of the new technology has not been overwhelming for application to the M242 medium caliber cannon barrel. In addition, costs for the Engineering and Manufacturing Development (EMD) phase of the implementation are expected to be high and may not be affordable.
The Manufacturing Readiness Level for machining the gun tube after explosive bonding the liner is not high. Waterjet cutting is the most promising approach, and it was used in the cost analysis as the least expensive approach. A waterjet cutter has demonstrated its ability to rifle an 84-inch long, 1-inch diameter steel tube. It is expected that by the end of 2015 it will have demonstrated the ability to rifle an M242 lined tube. At that point a case can be made to proceed to EMD.