Well friends, lets try this again shall we? Sorry this has taken so long to re-post after the unintentional deleting by a very apologetic Mod, but free time is a rare commodity for me these days. If you have done any looking on the internet though, you know how little useful information is out there on the M12, so I wanted to get this back on SGW for you.
This is a complete re-write of the original article. I have taken many new photographs and rely on them more, since “a picture is worth a thousand words.”
The proper part names from the Winchester diagram are CAPITALIZED, with the part number in parenthesis initially, and repeated as needed for ease of reference. The part and subassembly names are shortened where possible. For example, the complete RECEIVER EXTENSION (8)/BARREL (11)/MAGAZINE TUBE (21)/ACTION SLIDE (25) subassembly is referred to as the “EXTENSION unit,” and the GUARD-COMPLETE ASSEMBLY (45)/CARRIER (60)/HAMMER (50)/TRIGGER (55)/etc is abbreviated "GUARD unit.”
This article refers only to the original MAGAZINE LOCKING PIN (19) type gun, which is the classic M12. Much later in the production run the MAGAZINE was redesigned similar to the Remington 870. I will leave the description of that type for someone else to discuss in another thread, since I do not own one.
I only describe disassembly down to the point where the gun can receive a good cleaning and inspection, as well as some simple parts replacement if required. This task is well within the skill of any owner with a modicum of mechanical sense. The MAGAZINE however does have several complexities, which I will try to make simple for you.
Further work is best left to an M12-experienced gunsmith, or at least a mechanically skilled and knowledgeable owner with a fully illustrated repair manual. To go beyond would require re-staking pins and screws, which is a form of damage best not frequently repeated. It would also necessitate the proper readjustment of BARREL tension, and the reassembly of the fairly complex GUARD unit and BREECH BOLT. These areas can be cleaned and preserved intact with your favorite aerosol gun product.
Notice the staked screw and pin heads heads, and need for BARREL tension adjustment:
The gun is always referenced as being held level, upright, and viewed from butt to muzzle, regardless of the actual position of the gun at the time. A standard 12-hour analog clock code is used.
The M12 only requires a 3/16th” hollow ground standard screwdriver for normal maintenance. If you wish to remove the BUTTSTOCK (47) you will need a long-shank 3/8th” standard screwdriver to remove the BUTTSTOCK BOLT (4), and the proper tool to remove your particular BUTTPLATE (48) or recoil pad.PARTS OVERVIEW:
EJECTOR AND EJECTOR SPRING (30), BREECH BOLT (31)
GUARD SCREW (46), GUARD COMPLETE ASSEMBLY (45) with attached CARRIER (60), CARTRIDGE CUTOFF (29)
BARREL with attached RECEIVER EXTENSION (8)
MAGAZINE FOLLOWER (23), MAGAZINE SPRING (22)
MAGAZINE TUBE with attached ACTION SLIDE “bar” (25)
BARREL with underside “lug” shown (out of focus, shown clearly later)
MAGAZINE SPRING (22)
MAGAZINE BAND BUSHING (13), MAGAZINE BAND BUSHING SCREWS quantity two (12), MAGAZINE BAND (14), MAGAZINE PLUG (17), MAGAZINE LOCKING PIN (19) with inserted MAGAZINE LOCKING PIN SPRING (20)
MAGAZINE PLUG SCREWS quantity two (18)
MAGAZINE PLUG STOP (16)DISASSEMBLY:1. Gun safety:
Verify the BARREL (chamber) and MAGAZINE are empty of all ammo. Cycle the SLIDE. TRIGGER LOCK ASSEMBLY (54) engaged, i.e. safety—on. NOTE:
The ACTION SLIDE LOCK (58) must be released EACH time you wish to move the SLIDE rearward out of battery, unless the gun has just been fired with live ammo, i.e. dry firing will NOT release the slide as it does with some other pump shotguns.2. Unlocking the MAGAZINE:
The MAGAZINE is in the locked (ready for firing) position when the MAGAZINE, SLIDE, and BARREL are threaded through the EXTENSION, the EXTENSION unit is installed in the RECEIVER, and the MAGAZINE Locking Pin (19) is extended at the 11:00 position.
The BARREL keeps the PIN from rotating to the unlocked position as long as it remains extended at 11:00:
The MAGAZINE BAND BUSHING SCREWS (12) should remain tightened at this point. The MAGAZINE BAND (14) holds the MAGAZINE and BARREL together as part of the EXTENSION unit. When the SCREWS are tight, the MAGAZINE can rotate and slide snugly but freely through the BAND; only the BARREL is held tightly.
To unlock the MAGAZINE, the PIN is pressed downward until flush, so that it protrudes at the 5:00 position:
Using the PIN for leverage, the MAGAZINE is then rotated 90 degrees (one-quarter turn) clockwise, the PIN ending up at the 8:00 position:
This frees the coarse MAGAZINE half-threads from their counter-part RECEIVER half-threads, so that the MAGAZINE can be drawn forward out of the RECEIVER:CAUTION:
If the MAGAZINE does NOT rotate through the BAND, you may have rust, dirt, or damage under the BAND and/or the MAGAZINE/RECEIVER half-threads, and the MAGAZINE is frozen. You can try removing the BAND BUSHING SCREWS (12) and soaking both areas with penetrating oil. If it is still frozen after that, see an M12-experienced gunsmith as the MAGAZINE is thin-wall steel and could be damaged already, or end up damaged by you using excessive force.3. “Take Down:”
This commonly used term refers to splitting the gun into its forward and rearward halves, at the EXTENSION to RECEIVER joint, for ease of cleaning and transport.
This is similar to an O/U or SxS, if you think about it:
On the MAGAZINE there is a “flange:”
The flange prevents the unlocked MAGAZINE from sliding too far forward, which would cause the EXTENSION unit to fall apart rather than remaining a single unified forward half of the gun during Take Down.
In this picture the MAGAZINE and SLIDE are slid partially forward so that you can see the BAND, flange, and the ACTION SLIDE SLEEVE SCREW CAP (27) all in close proximity, but the MAGAZINE and SLIDE are not fully forward yet, and the gun is not ready for Take Down:
Slip the MAGAZINE and SLIDE as far forward as possible.
The SLIDE SLEEVE SCREW CAP will contact the rear of the flange, and the front of the flange will contact the rear of the BAND. The MAGAZINE and SLIDE are now fully forward and ready for Take Down: NOTE:
When the MAGAZINE is locked, the center of the flange is at the 7:00 position. When the MAGAZINE is unlocked, the flange is at the 10:00 position. Remember this reference point.
In addition to the flange, there are stamped MAGAZINE and EXTENSION “alignment arrows” which point tip to tip when the MAGAZINE is installed correctly and locked, at the 6:00 position:
When unlocked, the MAGAZINE arrow is at the 9:00 position, parallel to and below the SLIDE bar:NOTE:
The flange and arrow are your only fixed references on an otherwise uniformly cylindrical MAGAZINE. The arrow is hard to see when handling the gun, while the flange stands out clearly, so remember, 7:00 locked, 10:00 unlocked.CAUTION:
Ensure the SLIDE is fully forward and the SLIDE bar is held fully retracted from the RECEIVER, or damage may result to the bar and/or RECEIVER
when you rotate the halves of the gun:
There is a small cut in the side of the RECEIVER where you can see whether the rear tip of the SLIDE bar is still in the RECEIVER:
Or completely clear (ready to rotate and separate the gun halves):CAUTION:
During Take Down ensure both haves of the gun remain aligned and supported so that there is no side-load on the BARREL/RECEIVER threads:
Rotate the EXTENSION 90 degrees clockwise:
Separate the halves of the gun, again being careful not to damage the BARREL and RECEIVER threads.4. Separating the MAGAZINE from the EXTENSION unit:
Remove both BAND BUSHING SCREWS (12), allowing the BAND to be expanded. Place several fingers from each hand (no tools required) between BARREL and MAGAZINE, and pry apart, “snapping” the MAGAZINE and BAND off the BARREL, which requires minimal force. A little metal to metal scratching is inevitable.
Draw the MAGAZINE out of the EXTENSION, and separate from the BARREL, which will retain the EXTENSION attached to it:5. MAGAZINE PLUG (17), SPRING (22) and FOLLOWER (23):
These may removed be less often than normal cleaning, as desired.CAUTION:
The MAGAZINE PLUG (17) is under constant tension from the MAGAZINE SPRING, and will shoot out forcefully if not held in place once the PIN and MAGAZINE PLUG SCREWS (18), quantity two, are removed:
Depress the small MAGAZINE LOCKING PIN SPRING (20) with your small screwdriver tip to allow the PIN to be completely withdrawn from the PLUG/MAGAZINE:
Reverse the PIN and insert it partially into the PLUG/MAGAZINE so that the two PLUG SCREWS can be removed while the PIN holds the MAGAZINE SPRING tension.
Next, hold the PLUG in position with your thumb and withdraw the PIN. Gently allow the PLUG, SPRING, and FOLLOWER to separate from the MAGAZINE.
Slide the MAGAZINE PLUG STOP (16) off the PLUG by turning it, then removing it.
Finally, slide the BAND forward off of the MAGAZINE and remove the BAND BUSHING (13) out the side. I will discuss all these parts in detail upon reassembly.
The SLIDE must be left attached to the MAGAZINE at this level unless you have a special-purpose wrench to engage the SLIDE SLEEVE SCREW CAP (27) at the front of the HANDLE (28):6. GUARD unit, CARTRIDGE CUTOFF (29), and EJECTOR AND EJECTOR SPRING (30):CAUTION:
Though the inside of the average M12 RECEIVER is likely nowhere near as sharp as the inside of some more modern guns, due to the generous hand-fitting of these classics, know the inside of your gun before you go digging around in there with bare fingers. There is no accounting for previous wear and/or abuse.
Remove the GUARD SCREW (46) and gently lift the GUARD—COMPLETE ASSEMBLY (45) and CARRIER (60) unit, sliding them out and rearward. If it does not come free easily, you have either rust, dirt, or damage. The same applies as with the MAGAZINE BAND.
Turn the RECEIVER upright (open bottom down) and gently shake, hit with palm of your hand, or a soft tool if necessary, so that CARTRIDGE CUTOFF (29) falls out by gravity onto a soft surface:
Remove the EJECTOR AND EJECTOR SPRING (30) by gently prying it off the side of the RECEIVER with your small screwdriver.
It is a two-piece spring forming a single unit:7. BREECH BOLT (31):NOTE:
In order to remove the BREECH BOLT when not performing a complete Take Down, it is necessary to place the gun into partial Take Down. Unlock the MAGAZINE and draw it and the SLIDE fully forward until they both contact the flange to ensure the bar is clear of of the BOLT and RECEIVER. Only then may the BOLT be removed as described below.
Remove the BOLT by first pressing it with your finger into the top of the RECEIVER, against spring tension from the FIRING PIN RETRACTOR (35). Using your small screwdriver, depress (toward the BOLT) the rear
tab of the BREECH BOLT RETAINING LEVER (33). This is a “seesaw” toggle locking lever installed on the side of BOLT.
LEVER unlocked, for BOLT removal:
Slide the BOLT rearward and up, removing it from RECEIVER.NOTE:
The millwork inside the M12 RECEIVER is truly art in steel. This vertical channel permits the BOLT’s withdrawal:8. BUTTSTOCK:
Now that you no longer need the BUTTSTOCK as a handy handle, you can remove it if desired by removing the BUTTPLATE/recoil pad and using your long standard screwdriver to reach up to the BUTTSTOCK BOLT (4). Looking up with stock hole with a flashlight first will help you to get the screwdriver placed properly. Remove the BOLT, and slide the stock off the RECEIVER SHANK (2). CAUTION:
As with any gun, be careful not to damage the wood to metal joint.9. The gun is now disassembled for normal cleaning and maintenance.
I will leave each owner to his favorite cleaning products and techniques. I will however remind you that everything except the wood will rust if not protected, and that a small amount of grease on the BARREL and MAGAZINE threads will serve you well, as always. Also, do not let the wood get wet from cleaner/lubricant, just like any gun.REASSEMBLY:1. BUTTSTOCK and BUTTPLATE:
Reinstall at any time, in the reverse order it was removed.2. BREECH BOLT:
Check that the BREECH BOLT RETAINING LEVER is still depressed in the unlocked position (rearward tab toward the BOLT). Insert the BOLT into the RECEIVER forward end down, then slide in forward, so that its rear “hump” (not the FIRING PIN RETRACTOR) fits into the mortise in the top of the RECEIVER.
Hold the BOLT depressed under spring tension against the FIRING PIN RETRACTOR with one finger, then with your small screwdriver, depress (toward the BOLT) the forward tab of the LEVER, locking the BOLT in place:NOTE:
Think forward depressed (toward the BOLT) for shooting forward, rearward depressed (toward the BOLT) for removing the BOLT rearward, if that helps.
You can now let go of the BOLT and it will not move.3. EJECTOR AND EJECTOR SPRING (30), CARTRIDGE CUTOFF (29), and GUARD unit:
Install the EJECTOR AND EJECTOR SPRING. The curved spring portion goes against the RECEIVER wall:
You may prefer a hemostat or small screwdriver to get the part aligned, but you will need to gently work it into its mortise in the side of the RECEIVER with nothing under it, eventually.
Using fingertip(s) on top of the SPRING works well. Starting with it slightly to the rear of its mortise, compressing the SPRING flat against the RECEIVER wall with your right-hand index finger, then sliding it forward into its slot with your left-hand index finger, is quick and easy with practice. If it becomes wedged-in crooked, gently lift it out with your small screwdriver, and start over:
Install the CARTRIDGE CUTOFF by aligning the pin with the mortise in the side of the RECEIVER, the curved portion facing the MAGAZINE. It drops in easily and will stay in place by gravity while you install the GUARD unit, as long as you don’t bump it or jostle the gun unnecessarily.
Install the GUARD unit by sticking the CARRIER in first, then move it forward and up to seat it flush. Ensure it is fitted flush on all sides before installing the GUARD SCREW (46) at the rear. This requires only slight seating pressure, unless the RECEIVER or GUARD is damaged, or something is misplaced. When in doubt, back up and recheck your work.4. BARREL “lug,” MAGAZINE BAND and BUSHING:
Permanently attached on the underside of the BARREL is the lug. The MAGAZINE BAND BUSHING (14) and the BAND itself have mortises to fit over the lug:
The BUSHING is contoured to fit the MAGAZINE TUBE on one side:
And fit the lug on the other side:
Insert the BUSHING into the uninstalled BAND from the side and slide the BAND onto the MAGAZINE, ensuring the BUSHING mortise for the lug is facing up towards where the BARREL will be.NOTE:
Like the PLUG STOP tang, the BAND has a "gauge stamp" on it, as shown in the above picture. By facing that stamp also towards the butt, it will be concealed. Other than that, the BAND can be installed in either direction.
Because the SLIDE and BAND with BUSHING rotate freely around the otherwise uniformly cylindrical uninstalled MAGAZINE, it is necessary to remember the proper flange reference mentioned above, i.e. 7:00 locked, 10:00 unlocked.
When the MAGAZINE is unlocked, remember the MAGAZINE half of the alignment arrow pair is horizontal, parallel to and just below the SLIDE bar, at the 9:00 position. Because the arrow is very difficult to see, the flange is the easiest reference to maintain, as I said before. NOTE:
This will be the reference position for MAGAZINE reassembly and installation, as the MAGAZINE remains unlocked until the very last step of gun reassembly.5. MAGAZINE PLUG STOP (16) and PLUG (17):
Familiarize yourself; you can even practice putting this all together without the added difficulty of the MAGAZINE SPRING pressure, before moving on to the next step.
The PLUG has a mortise milled into it for the “tang” on the STOP. The PLUG is correctly installed in the MAGAZINE when the STOP rotates from 12:00 to 3:00 and the “gauge stamp” on the tang faces toward the butt of the gun:
Opposite side of the STOP and PLUG, no mortise, no gauge stamp:
Looking ahead for reference and understanding, visualize that once the BARREL and MAGAZINE are rejoined by the BAND, in actuality the STOP sits still, its “stubby forks” fixed on opposite sides of the underside of the BARREL, while the MAGAZINE and PLUG rotate as a unit 90 degrees, between unlocked and locked.
The MAGAZINE LOCKING PIN hole in the PLUG must be aligned on a 2:00/8:00 axis.NOTE:
I know this sounds unnecessarily detailed. However, it all goes together correctly only one way, while any combination of incorrect installations is possible and may not be noticed until several subsequent steps have passed. This is the most confusing aspect of the whole gun.6. MAGAZINE:
Insert the MAGAZINE FOLLOWER into the tube, open end facing the SPRING.
Insert the MAGAZINE SPRING. Compress it so the combined and correctly assembled PLUG and STOP can be installed.
With the MAGAZINE flange at the 10:00 position, rotate the PLUG until the STOP can rotate from 12:00 to 3:00.
Insert the LOCKING PIN partially as before to temporarily hold the PLUG in place. Now the SPRING pressure is off you.
Install the two PLUG SCREWS (18) and tighten.
Remove and partially reinstall the LOCKING PIN so that the tip is pointing toward the 2:00 position. Then depress the PIN SPRING with your small screwdriver and insert the LOCKING PIN all the way into the PLUG, leaving the PIN protruding at the 8:00 position. 7. Re-attaching the MAGAZINE to the BARREL and EXTENSION:
Insert the threaded portion of the MAGAZINE just into the lower half of the EXTENSION. As you are doing this, insert the SLIDE bar into the hole for it in the side of the EXTENSION. NOTE:
Both the MAGAZINE and bar will sit loosely, but they must remain inside the EXTENSION. If they fall out or rotate, and you reattach the BARREL to the the MAGAZINE BAND, you will have to take it all apart again.
Align the open forks of both the MAGAZINE BAND and PLUG STOP with underside of the BARREL, and verify the BAND and BUSHING mortises are aligned with the BARREL lug.NOTE:
It is easy to mess up this step because many parts have to be held in proper alignment simultaneously. Take your time.
Using hand pressure, grasping the BARREL and MAGAZINE with fingers and palms, “snap” the forks of the MAGAZINE BAND over the BARREL and lug, concealing the lug inside the mortise.
Install and tighten the BAND BUSHING SCREWS (12). The MAGAZINE should slide snugly but easily though the BAND, while the BARREL is held tightly.8. Rejoining the completed EXTENSION unit to the RECEIVER:
Ensure the SLIDE bar is fully forward, and flush with its hole in the RECEIVER EXTENSION: CAUTION:
Remember, this will damage your gun:
Turn the EXTENSION unit 90 degrees counter-clockwise until it and the RECEIVER are tight and perfectly aligned.9. Locking the MAGAZINE:
Verify the MAGAZINE LOCKING PIN is at the 8:00 unlocked position. Gently slip the MAGAZINE TUBE and SLIDE toward the butt until fully seated in the RECEIVER. Use the LOCKING PIN to engage the threads, rotate it from the 8:00 position to the 5:00 position. Press the PIN through to the 11:00 position.
Cycle the ACTION SLIDE to verify the BREECH BOLT cycles, the HAMMER cocks, and the gun will dry fire after again verifying the gun is unloaded.
Give the gun a final visual inspection, noting that everything is in its proper place:
The gun is reassembled and ready to shoot.A PARTING THOUGHT:
The M12 is a hand crafted work of mechanical art, designed long before the term “foolproof” was coined. It is built of nothing but the highest quality machined steel and American Walnut; there are no stampings, plastic, or aluminum. With a little knowledge and normal maintenance, these fine guns will long-outlast all of us, and our children and grand children too. A great many of the two million M12s built have already outlasted our parents and grandparents who purchased them new.
It is sad however to see how many badly neglected M12s are out there. As you can see from my "up close and personal" photographs, my recently purchased $325 M12 "shooter" was badly neglected from a cosmetic standpoint for many years, which is a travesty. Mechanically however, it is a low round count gun with a with a million cartridges yet left to be fed it.
These guns are too precious to be neglected because owners cannot figure out how to take them apart, clean and protect them, and put them back together.
It is righting this wrong to which this article is dedicated.