Need help with an 1187 SuperMag

Hello,

I bought an 1187 3.5 inch Supermag over on the auction side. It ejects and feeds the big shells OK until the last one is fired. The carrier appears to come up too far and stops the bolt from closing.

Also, they sent me the extra ring which I thought was for the lighter loads, but I'm not sure if it goes first or last on the tube.

Thanks for any thoughts you might have.

KC