higher spring rate was achieved, and this was the solution for the bulk of the misfires. The weapon, when cold, would fire at about 650 rounds per minute (rpm). When it was very hot, after several hundred rounds had been fired, the cyclic rate climbed as high as 1,100 rpm. The mechanical functions of the weapon just would not work properly at these high rates. There were several problems that occurred at high RPM, including link separation. A "rate regulator" similar to that used in M60 machine gun was installed in the buttstock. It was basically a shock absorber (designed by Cadillac Gage Co) that eliminated the recoil bounce off the back plate. This allowed the main drive spring to control the forward velocity of the carrier. With the "rate regulator" installed the cyclic rate of the weapon was limited to about 800 rpm. This resolved the mechanical malfunctions that occurred above 1,000 rpm. In combination with the "rate regulator" changes were made to the belt feed system for better reliability. 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 5,000+ 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. Several weapons with these improvements were produced by Cadillac Gage for testing by the Army Materiel Command. This would be the last chance for the weapon, now designated XM207. With its latest design changes, the weapon passed all tests. But soon afterwards, president Nixon announced that the United States would be withdrawing from Vietnam, eliminating the urgency to adopt a new infantry weapon. The XM207 project was cancelled. Deployment Even very early on in its development stage, a small number of weapons were hurriedly pressed into service with the U.S. Navy SEALs deployed to Southeast Asia. During 1967, the Stoner 63A system was field tested by