Here's a question for you. What kind of powder where you using when the bullets where tumbling. We had the same issue with the Hornady sst bullets. Both me and my buddy where shooting the same gun's, bullet, and caps. The only difference is I was using the 777 pellets and he was using the 777 loose. My gun would tumble and his wouldn't. We figured out that because the pellets where designed to be used with 209 ignition, they where causing inconsistent pressures. To keep it short we stopped using the pellets and have never had a problem since.