Another issue was low gas pressure when firing tracer rounds, causing failures to cycle. The addition of an expandable steel sealing ring in the gas chamber kept the external gas leaks to a minimum, retaining enough gas pressure to cycle the action reliably. The next issue was the polycarbonate buttstock being unable to withstand the 5000+ pound recoil force of firing anti-tank grenades. With the weapon's tight weight restriction, using a heavier metal buttstock was not an option. Several ribbed designs were evaluated, but none were successful. The solution was to pressure-fill the hollow buttstock with a lightweight urethane foam, imparting structural rigidity and high impact strength. This solved the problem without going over the weight limit, and made the buttstock extremely durable.