These instructions are specifically for the M1100 series with solid stock but are entirely applicable to the M870 with solid or folding stock. How well they apply to the new magazine fed 870s I can’t say.
1. First thing is to push out the pin that holds the cocking handle on. This is really tight and is a real pain to get out. All I can suggest is that you find something long and thin and bash it repeatedly. After I knocked this pin out on my M1100 it was never tight again so I replaced it with a screw.
Then you’ll need to pull the handle off. Again, it’s extremely tight. Just keep pulling/levering and it’ll come off. Try to avoid too much waggling as I suspect this could damage the pin in the bolt.
2. Remove the gas resevoir and undo the screw in the back of the stock (a long screwdriver or an extension makes this much easier). Slide out the gas resevoir receptacle and you’ll see the pin that attaches the gas hose. Pop this out and detach the hose. You can now remove the stock.
3. On the left side of the outer barrel is a small grubscrew – remove this. (be careful when replacing it as it threads into the plastic outer barrel and this is very easy to strip)
Remove the barrel/magazine clamp and unscrew the magazine extension. Remove the spring and let the magazine follower slide out of the magazine tube. Now you can pull off the barrel shroud and foregrip.
4. At the back of the receiver (both sides) are a pair of Allen screws – these need to come out. Also, push out the pin that is just ahead of these – it slides out easily in either direction. Now you can pull off the trigger guard (slide it downwards).
5. Now the receiver shroud will come off – slide it backwards (push the shell lifter in slightly so it doesn’t trap the shroud) and when you get to the trigger rotate the back of the shroud downwards.
6. There are two metal fittings around the point where the barrel mounts. Undo the two screws (be careful not to lose the nuts from the left side) and remove them. There is a small pin inside which secures the barrel – don’t lose this (I managed to lose mine). Now pull out the barrel.
7. Undo the two larger screws and the four little screws that secure the magazine tube. The plates that are secured by the little screws need to be bent outwards a little to allow them to slide forwards.(I’d recommend bending them as little as possible) Now you can pull the magazine tube off.
8. Under the plates attached to the magazine tube are the two shell latches. Remove these (they fall out quite easily) and make sure that the springs are present. These are not very well attached so take care not to lose them.
9. Remove the two screws (marked). Now you can lift off this side of the receiver.
10. Lift up the back of the mainspring guide (use a small screwdriver for leverage) and remove the guide and spring. Also, remove the trigger (there is a small spring on the underside of the top bar – keep an eye on this).
11. Now you can remover the action from the receiver. When you reassamble, you need to locate the brass tube (marked) into the recess on the receiver.
Now that you’ve got your M1100 in bits there are a few things that you can do to improve it’s performance.
First, plenty of lubrication is important. Make sure that all of the bearing surfaces (trigger/valve contact point especially) are greased and all of the points of contact for the bolt should be well lubed.
The biggest weakpoint on this gun is the gas hose. The plastic is quite rigid and kinks very easily. I have replaced the original hose with some water hose (from my local auto parts shop) which is much less prone to kinking and it has made a world of difference.
I sometimes find shells sticking just inside the magazine tube. While some of these have been due to shells with slightly deformed rims, the problem is largely due to imperfections in the receiver at the point where rounds exit the magazine tube. A bit of filing and polishing here will fix this.
The metal of the shell latches is quite soft and after a fair bit of use they spread at the front creating a lip that catches on shells. This can also be fixed with a bit of filing and polishing. This is going to be a recurring problem and in the long term I suspect that I will need to replace the shell latches with something made from better metal. I’m going to look into whether real steel shell latches will fit.
Finally, I have removed the ball bearings and springs that make the trigger snap in and out of position. This makes the trigger pull much lighter and stops it from jamming (which has happened a couple of times). If you do this then you can no longer ‘squeeze’ the trigger as this will cause gas to leak and, most likely, a mis-feed. As long as you always pull the trigger sharply this is a good modification.
Shortening the M1100/870 series
If you want to shorten your M1100/870 it’s really quite easy. First, you need to remove the magazine tube extension and cut it down to the required length.
Of course, you’ll need the plug out of the end (which you have now just cut off). This comes out to the front. The easiest way to do this is to hold the magazine tube in your hand (plug to the bottom), drop a long screwdriver into the tube and hit the top of it sharply with a hammer – this will pop the plug out. Now you can hammer it into the front of your shortened magazine extension.
Now the outer barrel can be cut down to match. If you find that the inner barrel protrudes, it’ll need to be shortened as well. Try and do this neatly (small pipe cutters can be had quite cheaply from DIY stores) but the M1100/870s aren’t exactly precision weapons so don’t worry too much.
Finally, the magazine spring will need to be shortend in proportion to the shorted magazine tube.