ORIGINAL: bronko22000
Bigcountry - I know you are pretty knowledgeable about firearms from reading your post. But my load for my guide gun is 51 gr of H4198 behind a Hornady 300 gr interlock. I've been shooting this load for a few years without any noticable bulging of the cases. You may want to contact Marlin as you may have a chamber issue.
I am knowledgeable about bolts but not lever. Its a totally new world to me. I am going to look at all my old brass and measure it. Especially the 41gr loads. If it has the same bulge, I might get the chamber casted or send in. Could be dangerous I guess.