This testing was conducted in 1980 or 1981 at FLETC in Georgia. The tests called for the guns to shoot 10,000 rounds of full magnum (RP 125 and 158 jhp IIRC) with no more than 2 of maybe 20 minor malfunctions. This included ejector rod, rear sight mounting screw, and loose front sight. IIRC the specs called for fixed or adjustable sights. S&W submitted the Model 66 and Ruger the Speed Six. The guns were random samples from the production run. S&W was tested first as they were the low bidder.
The longest a M66 lasted was 1500 rounds when the timing was so bad it wasn't safe to shoot. Others went out with broken bolts, hands, and other parts. The first went out at 400 rds when the cylinder exploded. This was all factory ammo so reloads weren't to blame here.
The testing on the M66s was done before I got into it. I did see and examine the guns that failed. I helped fire the Rugers.
The way the test was conducted, we fired as fast as we could reloaded and fired again. We wound up taping padding on our hands and wore a glove on the left as the cylinder was so hot. These guns were really abused. They got cleaned every 500 rds. We had solvent sizzle off the cylinders they were so hot.
The Ruger's lasted 10,000 rds with zero malfunctions. It was decided to continue shooting them to see how many rounds they would last. One Ruger went out at 13,000 rds with timing issues. The rest of the Rugers went to 20,000 rds with no malfunctions. The bean counters didn't want to pay for anymore ammo so the test ended there. The Rugers obviously won.
The L frame was already out but I think S&W wanted to dump these M66s on USBP. I'm sure the L frame would have fared better.
I also prefer stainless guns and have both a M66 and Speed Six. Based on my experience I don't shoot many magnums through the 66.