Head, I find it hard to imagine the gas check is the cause of inaccuracy in your Marlin. That's a real puzzler? Have you been able to try sizing your bullets to different diameters. I've shot cast bullets in several Marlin's and never had any problem with accuracy and gas checked bullets...at least in .357 and 30/30. I had a 44 Magnum that didn't much like anything, but as I started oversizing my bullets, it tightened up a bunch.

I'm betting your problem is bullet diameter...not gas checks.