Disconnector:
Here we get to probably the most complicated and trouble prone aspect of the mechanism. Disconnectors are always a difficult part of FCG design, since they operate based on the violent energy released by the cartridge, but act upon delicate trigger group elements tied directly back into the user's finger. Done wrong (CZ52) and they will beat the hell out of your finger tendons --trigger slap. Done right, and internal parts will shift to deactivate the trigger/sear interface in such a way that it is undetectable. Done really wrong, and the sear won't disconnect (hammer follow or out of battery events)
The disconnector (#1, "disco" if you're into the whole brevity thing) is a punched and folded sheet metal piece. Incredibly cheap construction for something that is 1) critical to proper/safe function, 2) is wrapped around and dragging over every other part in the fire control group (FCG) in this particular design, and 3) has a very inefficient design in terms of friction and fit-sensitivity. It is roughly U-shaped, two flat faces joined by a bridge at the front, with two very short "hooks" at the bottom, and two "ears" at the top. The ears are rounded cams which are acted on by the slide, the hooks link up with the transfer stirrup to disconnect the trigger from the sear after firing. The whole schma-goigle actually slides vertically on two slotted holes through which the sear and hammer pivot pins pass (much more on this later)
When the slide is fully in battery (or at specific points during the cycle where the action is too far out of battery for the hammer to hit the firing pin) the ears of the disco are not in contact with the slide, and a fairly stiff return spring drives it upward. When is free to move upward, the hooks at the bottom of the disco pull on mating hooks on the transfer stirrup, working to pull that piece upward. The stirrup can only move upward and in front of the sear tail if the trigger is released, however. When the hammer is fully cocked, and the trigger fully reset, the disconncector is fully raised, thus checking the slide is properly in battery and making the gun safe to fire. Once fired, the slide flies back out of battery, and strikes the ears of the disco, forcing it downward along the slotted holes. As the disco shifts down, the hooks on its lower end drive the rear end of the transfer bar down, pushing it off the sear tail. At this point, the action cycles, and the sear re-engages the hammer, without trigger input. Only when the trigger is released can the transfer bar slip back over the sear tail and start the process over.
This concept of knocking the transfer bar off its sear engagemnent to deactivate the trigger is how my CZ52 works, though it is accomplished by a lump on the bar being struck by the slide (hence the slap) rather than by a separate piece. The linear motion of the disco in this gun is why no slap is transmitted; the disconnecting force is perpindicular to your finger's force direction. It's a clever design, probably used in pistols before, and for sure used in some rifles. But, as I'll get into later, it is very dependent on high quality parts, of which the R51 has few, which means the action is less than stellar.
Potential Issues:
-The hooks on the disco are little more than burs. Rough burs. SHARP burs. Not sure what Remington was expecting to happen when you slide raw shorn edges across each other. If these hooks were better defined when formed and were more like little tabs (or whatever the opposite of a rabbet is ) the transfer bar would slide back & forth on them infinitely more smoothly.
-The slotted holes in the disco are stamped rather than machined. I know punches are cheap, but you really need machined surfaces in a fire control group. This is basic stuff. The holes are inconsistently oversize (multiple punch passes formed the slots, the material shifting and deforming a little each time) and the material around them is significantly cratered (dull punches). At least the holes were debured, well, probably because Remington knew the guns would go Kaboom or full auto if they didn't.
-For a part that depends on precisely tracking in a linear path, it is very poorly constrained to this motion. The loose holes allow it to shift/rotate forward & backward to the extent that it looks to a casual observer like myself to be a pivot on an axis further down in the gun, and they also allow it to pivot left to right if the ears are not loaded up by the slide symmetrically (and why would they be on such a loose gun?)
-The disconnector is also very loosely constrained side to side; a good .02" to .05" of play to its sides allows it to both slide to the left and right, and also more importantly allows one side to drop before the other if the ears are struck assymetrically, rotating the part along the barrel axis.
-When, not if, the disconnector does stray from its proper linear path, it readily contacts both other trigger group parts (hammer, sear, transfer bar) and the frame walls with rough, broad surfaces, and sharp bur-covered edges. The barrel-axis pivot is what causes the whole works to bind up almost completely, and is my theory for the stuck guns out there; one ear was a little too tall and got pushed down more than the other side
-The exact angle at which the slide contacts the ears is critical to determining how efficient the slide motion is transferred to the vertical disco motion. If the disco's slots are oversize and it rises too high, the rounded front contour moves up higher, and the slide hits something close to a vertical wall and damages the ears' surfaces or the disconnector itself. Likewise, if the slope is too shallow, it will not drop enough and the hammer will follow. I think that if too much force is applied, regardless of how efficiently it is delivered to move the disco, the action will bind, so it is critical that slop be eliminated as much as possible so the ears don't need to move as much to take it up (and the cam angle can then be as low as possible)
-In its fully raised position, I think the disco hooks actually bind up on the transfer bar between its hooks and the tail portion that pushes the sear tail. If not be design, it is due to the geometric discrepancies from all the burs here.
TCB