Is the gun still under warranty? Do you have a reliable smith nearby?
I've had a bunch of Marlin's in 45-70 (including a Guide Gun)and nothing has ever showed up of that nature... and I load 'em pretty hot! The barrel on the Marlin is pretty heavy, with lots of metal, so it's got to be, as you say, a "sloppy chamber". If you bought it new from a dealer, he should replace the gun once it's verified there's a problem with the chamber.
My current Marlin is the "classic" with a 22" pipe and it had a fault from the factory in a bent magazine follower which would not elevate the cartridges to the chamber. It went to my smith and he straightened it, saying it was a fairly common fault with Marlins. It's worked great ever since.
A faulty chamber is another matter entirely. Sorry to say it, but the gun will need to go back to Marlin, or at least to your dealer and hopefully he'll do the right thing.
Bob
www.bigbores.ca
