I had a problem with my 500 similar to yours.... the gun had about 60 factory rounds thru it and I loaded some mid range loads using 325gr with LilGun, W296 and AA5744 to try.
After a few shots my cylinder got hard to turn and started to lockup often. I also couldn't get it to open easily. I thought the ejector rod had come loose but that wasn't it. This was intermittent - to all the loads.
I noticed the primer indents looked uneven and discovered half of my firing pin was fractured off. I sent the gun back to S&W and they fixed it promptly. They really didn't say what was wrong but the repair ticket indicated they replaced the Cylinder and Yoke along with the firing pin and bushing. All I can think of was that something was out of spec or got sprung somehow.