This is Dads' rifle, and it had a problem: put in a magazine, and cartridges wouldn't feed from it to the chamber unless you pushed up on the mag; it was slightly out of position. And no, it wasn't just that mag; he has several, and it's the same with all. So, how to fix?
The magazine guide is a L-shaped bar that screws to the bottom of the receiver, and it's pretty straightforward to change the shape just a touch. Here's how I did it.
Note that "This is how I did it" part. I'm fiddling with a used rifle with a fairly simple problem; if yours is new, you might want to just send it back to Marlin to be dealt with so they won't tell you you've voided the warranty. Like anything else, it can be remarkably easy to screw up and simple job and make things worse. So if you try this, don't blame me if something doesn't work the way you think it should.
Step 1 is removing the receiver/barrel from the stock. First, open the bolt and make sure the chamber's clear. If the magazine is in, take it out. Check the chamber again. Might as well remove the bolt while you're at it.* With all clear, you have to pull two screws
Once out, turn it over. You'll see the magazine guide and latch assembly at the rear of the magazine well, attached by two screws(white arrows).
Checking the piece with a machinist square** showed it to be a few degrees short of being a right angle. I figured reducing that amount by about half would do it.
Note and Warning:
I don't have any .22 Magnum action proving dummies, so I had to use cartridges. Which you have to be damn careful with. What I did was put a couple of rounds into the magazine, and slide the bolt forward to see if it picked up the cartridge and fed it into the chamber properly; I did not lock the bolt. Pull it back to eject that one, push forward to feed the second, then straight back to eject. It was NOT necessary to lock the bolt to see if this worked, and if you don't lock it there's no way to accidentally bump the trigger or have some other "How the HELL did that happen?" leading to an unintended loud noise and damage.
If you're working on something and have to fully close & lock the action to check the work, either buy some dummies or make some; and make damn sure you're using the DUMMIES when you put them in the mag or action. You have been warned.
One thing I did find on this you need to watch: the assembly can rotate a few degrees to either side until you lock the screws all the way down, and if it's too far off center the mag body will run into the receiver, making it either difficult or impossible to fully seat the mag. So I left the screws just a touch loose, installed a magazine(yes, making sure it was empty first), made sure it was centered, then locked the screws down.
It worked. I took it back down, made sure all was clean and wiped it all lightly with Eezox, reassembled, and it was done. I fired it this morning, and it worked just as it should.
One more thing: the trigger on this was pretty heavy, so while I had it apart I took the opportunity to disassemble the trigger and replace the trigger spring, same way I did in this post a couple of years ago on a Remington. The trigger designs are different, but both use a coil trigger spring, so I gave it a try. Made a big difference.
One job accomplished successfully.
Take another look at the bottom of the receiver
*Yes, I left it in.
**Can be a very handy tool, and can be found for not very much. For instance.
***Staking is taking a centerpunch and actually driving some metal in the dovetail against the piece in question, so it can't move. If you're going to do this, make certain the piece is properly in place before you do.