Evolution of the 870 breech bolt and slide assembly

[redgoat]

This is more of a "here's what I've observed" post than a how-to or an open ended question.

While researching all of the parts involved in the "flexi-tab conversion" for an older Wingmaster I learned that there is more involved than just switching the shell carrier to the new flexi-tab one. As I understand now, there can be up to four parts involved. I've listed them below along with their corresponding parts diagram number

1. The flexi-tab shell carrier (part # 7)
2. The newer breech bolt with relief cut (part #4)
3. The newer slide assembly (part number 57)
4. The forend tube assembly (part # 28) (I believe that this is optional, but probably a good thing to include if making the conversion. However, note that a flexi-tab forend tube assembly will not work with the pre- flexi-tab breech bolt and slide assembly without modifying one of the slide rails on the tube assembly. I will post some pics which show why this is so).

I found that the various 870s that I have access to cover a surprising number of variations in these parts and so I have prepared a series of photos to document what I observed.

To get things started, here are some side-by-side photos of the variations I have noted.

Bolt / carrier #1 is from an old well preserved Wingmaster that had a 28" fixed choke (modified) ribbed barrel.
Bolt / carrier #2 is from an _old_ 870 that appears to have been used by an LE organization. It is a Wingmaster, not a Police model, however.
Bolt / carrier #3 is from a recent Police Magnum model which apparently came with the flex-tab conversion.
Bolt / carrier #4 is from a new (2015) 870 Express, added for comparison, a "flex-tab conversion baseline".

image.jpeg (419.47 KiB) Viewed 9253 times

So, to summarize, the first two examples are pre- flexi-tab, and the last two are recent that have the flexi-tab setup.

I will post additional comments in this thread to contain close-ups of the parts in question.

I also found at least one other variation but do not have pictures taken yet.

If you have information regarding when Remington transitioned from one of these versions to the next, please share it here with everyone.

[redgoat]

Details on bolt / assembly #1. This is the "oldest" version of the breech bolt and carrier assembly that I have come across. There may be versions which precede these but I've not seen any. I have access to a couple of 870s (including the one from which these are in) that date back to the 1950s. If anyone has more information on these, please let us know!

UPDATE # 1
I've found a bit more of information about the technical jargon surrounding the breech bolt & slide assembly in The Remington M870 and M1100/M11-87: A Shop Manual, by Jerry Kuhnhausen, 1992 edition, still available through Brownell's. From this point forward, I will just refer to the book as "Kuhnhausen" and give the page number from the 1992 edition where I found the information.

So, Kuhnhausen (page 70), reveals that the "slot" on the bottom of the slide assembly (he calls it the slide block) is called the "carrier dog cycling slot." Its placement and level of wear and tear affects the timing of the shell carrier rising. Mystery number 1 solved.

On the same page, Kuhnhausen also reveals that what I have labeled as a "lug" on the top side of the slide assembly (aka slide block) is called the "locking cam".

If there's enough interest (on the part of you who read this and on my part also), I will try to work up and post a pictorial glossary of all this jargon.

breech_bolt.jpg (348.81 KiB) Viewed 9217 times

slide_assembly.jpg (431.16 KiB) Viewed 9217 times

[redgoat]

This second example breech bolt and slide assembly are from a heavily used but well cared for Wingmaster 870 that dates back to sometime (probably) between the mid-sixties to mid-seventies.

The most noticeable change to the breech bolt is the addition of a cut-out and pinning mechanism for the locking bolt. The locking bolt will no longer drop free when the breech bolt and slide assembly are removed from the receiver.

Changes to the slide assembly are the elongation of the locking bolt lift lug (the proper name for this lug is "locking cam") and the movement of the slot on the bottom further toward the front of the assembly away from the tail. By the way, can anyone chime in and help us figure out what the function of this slot is? (UPDATE: Never mind. It's called the "carrier dog cycling slot" and its name accurately describes its function.)

UPDATE # 1
I have found the information I was seeking regarding the technical jargon for the mystery "slot" on the bottom of the slide assembly and the "lug" on the top of it. See my prior post.

sm_breech_bolt_view_01.jpg (147.33 KiB) Viewed 9220 times

slide_assembly.jpg (370.64 KiB) Viewed 9216 times

[redgoat]

As promised, here are the annotated pictures of the flexi-tab version of the 870 breech bolt and slide assembly.

First, the breech bolt:

breech_bolt_view.jpg (122.97 KiB) Viewed 9156 times

Now the slide assembly:

slide_assembly_flexi-tab.jpg (279.67 KiB) Viewed 9156 times

I think that the annotations are fairly self-explanatory, but here are my additional observations:

1) The carrier dog follower slot is now "closed off" from the edge of the slide assembly, but also extends completely through the base of the assembly, creating a "peek-a-boo" hole! I suspect that this was done to enhance the strength of the assembly. The Kuhnhausen shop manual instructs gunsmiths to inspect this area for stress cracks and to replace the slide assembly if any are found.

2) The additional notch to engage the left carrier rail of the forend tube is interesting. Its presence does not preclude the usage of the flexi-tab breech bolt and slide assembly in combination with an older pre- flexi-tab forend tube assembly. However, if you must replace your old forend tube assembly with a new one, you will have to either replace your older slide assembly or grind off the engagement lug on the left rail of the new forend tube assembly.

I am not going to post any detailed pictures for the Express breech bolt / slide assembly (#4 in the original "top" post) because it is identical to the Police Magnum pictures in this posting.

I will wrap up with some additional observations and pictures in the next post.

[redgoat]

Wrap-up and additional observations.

[BurstBarrel]

Awesome post. Looking forward to learning the whole package on history and conversion parts.

That old Wingmaster with the fixed modified choke sounds like a very nice gun.

[Vitaly]

Very interesting post! Thank you!

[Synchronizor]

According to the Kuhnhausen manual, 870 slide assemblies have through slots for engaging the carrier dog, while early slide assemblies for the Model 58 had grooves instead. The manual really doesn't go into much detail on this though.

[BurstBarrel]

...

If there's enough interest (on the part of you who read this and on my part also), I will try to work up and post a pictorial glossary of all this jargon.

...

Since we are into the details don't stop now. The pictorial glossary sounds like a great idea and if possible the function of each part and its slot, cutout, etc.

[redgoat]

Thank you for the encouragement, BurstBarrel! I will see what I can pull together. I am certainly not an expert on the 870 by any stretch of the imagination, but such projects are very enjoyable and I always learn more while doing them. I hope that others find my self-educational musings of value too.