Can the Marlin Lever 357mag be opened up to 357 Max and the Max feed without any other work ?

no
BTW the .357 max operates at CF rifle pressures and is way over pressured for the Marlin.

So even if you figure out how, Don't.

Thanks for the info Rusty, I found the same info on other sights. Guess if I want a 357 Maximum in a rifle its going to be the NEF. Then again, the 357 mag Marlin has a good feel to it. Decisions Decisions, Decisions.....what to buy next..The marlin comes in a pound lighter than the NEF, load 9 rds in the tube and your there. I already have a 35 whelen and 38 S&W so the 357 mag is a good fit for me as a reloader....

The 357 mag now 35,000 psi,
The 357 max is 40,000 psi.
Not a big deal for a handloader.

The 357 mag is 1.59" over all length.
The 357 max is 1.99" over all length.
This is a big deal for a lever action.


In the NEF the length not an issue.
In the NEF feeding is not an issue, so sloppy SAAMI chambers are not needed or desired.
If I were to ream out the NEF to 357 max for a handloader, I would not use a standard reamer. I would save money and accuracy and use an industrial reamer, a .380" straight fluted reamer [or what ever .001" size the brass comes out of your sizer] and a throater.