why she will work fine when manually cycling the action as opposed to working on its own
The lifter, part 46, works much slower and with less force when cycling it by hand, than it does when fired. You will see the lifter down inside the magazine guide. It pivots up at the rear, and works off a spring, that holds it in place. There is a cam on it, that the bolt hits, when going back, that makes it push the shell up, plus cuts the one off in front of it, in the magazine tube, when the bolt is back far enough. If the guide lips are apart too much, on the guide, it will force the cartridge up too much at the front, over it.
The force from hand feeding to firing, is about the same difference as tapping a nail with a hammer, and pounding on it. That is why I wonder if it may have sprang open, with a spot weld broken. That, or it has opened up gradually over time. Either way, the part can be fixed.
If I remember correctly, this magazine guide acts similar to a Harrington & Richardson box magazine design, in one of their .22 bolt actions that they produced. The lips are at the rear of the magazine guide, instead of at the front. Those hold the cartridge in line with the chamber, until the bolt has started the bullet into the chamber, then the rim comes up out of two narrow slots, one on either side, as it is pushed on into the chamber. The Ruger guide works similar to this, except it is a rotary magazine.
On the H&R, they had their box magazine held together, by peening over locking tabs on the four edges, similar to riveting. They would work loose, allowing those lips to open up, and you would essentially get the same problem. I used to put a small tack weld on them, after I closed the magazine body back together with a C-clamp, then grind them flush.
On the 187, and the older models, that guide is formed, by rolling the sheet metal into a tube at the front, and there is a seam at the top, where it fits to the magazine tube. That is where it might be spreading open. Also, make sure the mounting screws are tight, that holds it onto the bottom of the receiver. Those screws hold the guide together, and stop it from moving, at the rear.
If you can't get it to work, and since Numrich is out, take a look on eBay or Guns America for the part, as you can sometimes find those parts there.
