I have heard of a part that when it breaks (if it breaks) the Marlin will not function. I currently own two (see my picture of the Marlin 120s (One has a 38" barrel that I had a Choke Tube system installed) the other is a true 40". I love them and they both are in great working order.

I did a bit of research (duckduckgo) and found a reference on another site (don't want to cross link sites) where the problem is briefly described.
I'm not sure that your symptom is this major flaw.

I hope this helps. Do you have a reputable local gunsmith that could take a look at it for you?

Maggs

 

When I took it apart to clean it hoping that would fix it I did not see anything that looked broken and the gun fires just fine but then it locks up and it only locks up when shooting. I can pull the slide back and forth using the button with the gun empty

 

Hmm.. So, let me ask a bit about the ammo. AND describe a situation that occurred years ago when Steel Shot was coming on the market.

Our family was shooting Remington 870 Wingmasters and Remington 1100s in the duck marshes. Steel shot was coming on the scene and it was expensive. Federal and Remington would work fine in both. Winchester was a bit less expensive, BUT... on occasion, after firing a shell, the cheap "BRASS" (probably tin/steel) would NOT allow that hull to be extracted. So, we had to (field strip - ie.. take the barrel off to remove the expired hull)..
It was only with the Winchesters shells.

Have you noticed with different ammos?
Don't do this in the house.. (only do this with the gun pointed in a safe direction) BUT, can you cycle shells through the gun using using the release button to 'unload the magazine tube?'

Just wondering if it's an ammo issue.
Be SAFE!

Maggs

 

I have used all kinds of amo from duck loads target loads pheasant loads turkey loads and all different brands and it’s the same every time and it cycles just fine with amo in it without shooting

 

So, more questions. This may require some time at the range and posting a few pictures.

When this occurs, you have to "slam the slide forward". Can you take several pictures before "slamming the slide forward"?

Is the action, "coming out of battery" at the shot? (starting to eject and opening the action ever so slightly?) Maybe take a picture of the bolt? Here the bolt is still in battery position.

Also, before slamming the slide forward (when you notice that it won't eject, take the barrel off (the shell is already fired, so there is only an empty hull) and take a picture of the slide from the top down so that we can see the magazine tube and where the forend rides on the slide? Might even need to get a meaurement or two on the downward picture.

 

it cycles just fine with ammo in it without shooting....

Are you saying that it will cycle unfired shells, but sticks if you fire them?

- Test: Will it fire and properly cycle when using brass (verified brass*) base shells ?


* (verify w/ magnet)

 

Just thinking out loud here. Could it be that upon firing, the pump arm is being slightly pulled back so that it is tight (jammed) against the action lock? Pushing the fore end forward would alleviate the pressure and allow the lock to reset. Again, just thinking.

 

I see that this thread is over a month old, but just curious. Is this gun new to you or have you owned it for awhile? I once owned a 120 Magnum years ago.

 

Well, the OP hasn’t been back since the day he joined with this thread/question. You may get an answer, you may not. But I would suspect, he figured it out or moved on.

 

I tried this with snap caps, that's exactly what it is, I repeated the process again and it locked 100 percent of the time i pulled back wile firing, with mine I only had to apply a small bit of pressure forward and it would unlock. Though my 870 didn't care either which way.