Might start with a cut up soda can then work up to larger shims.
I had the spring raised about 3/16 of an inch. Those cartridges just keep slip sliding around. More shims until it stops? My BIL has the same gun, so when he gets back in town I'm going to take a look at the set-up of his floor-plate/spring/follower. Other than spring tension, I'll see if there is any difference.
A temporary fix might be this: Since it's the base of the cartridge that makes the sound as it settles against the aft side of the magazine box, I might just attach a small strip of my wife's very thin craft foam to the back of the box with double sided tape. There's enough room between the back of the follower and the back of the box so the foam won't interfere with anything, and that should keep things quite until I can either figure this puzzle out or send it off for someone else to figure out.
