Failure to extract - forcing cone fouling

[tom]

Tried some SB #1 magnum buck today and had an FTE. Hadn't had any issues with my 870 Wingmaster earlier so I was little surprised. To get the empty shell out, I had to literally hammer the shotgun down to get the extraction happen. Now, I'm quite new to 870 - but I had read about how to handle the jam so it wen't fine.

When I cleaned the gun back home I noticed there was crazy amount of crud collected about 3-4 inches into the barrel. I had to scrub it with a brass brush like crazy to get it all out and noticed there's grooves/threads at this point in the barrel where all the crud gets stuck.

Are these grooves something that I should polish away to avoid the crud build up at that point? I don't understand what purpose do they serve at that point in the barrel, because they pretty much just make the dirt build up faster.

EDIT: After some googling and reading - I'm guessing my issue is excessive fouling in a rather rough forcing cone in the barrel. My question still remains, can the forcing cone area be polished to avoid excessive fouling?

Here's a picture of the grooves (28" LC Wingmaster barrel) after I cleaned it:


And here's a pic of the poor buck shell. I'm guessing when it was fired, the shell opens up and being magnum - it opened up all the way to this grooved area with all the crap in it and welded itself there.

[MStarmer]

You should be able to polish till your hearts content on that thing. Thinking about it since the back bore and extend forcing cones all the time I can see how you could do any damage. If you follow AI&P tactical's thinking you will polish the chamber / barrel each time you clean (0000 Steel wool). There's no rifling to worry about so I would make that thing smooth as glass first with steel wool and a drill followed up with some polish.

http://www.aiptactical.com/Page_2.html

[Synchronizor]

I don't think the forcing cone is what caused your extraction problem. Assuming your gun has a chamber manufactured to spec, a 76mm shell shouldn't unfold beyond it into the forcing cone (unless your gun actually has a 70mm chamber, but I think you would have noticed that).

Cartridge & Chamber Drawings - 12ga 76mm (3") - SAAMI

12ga 76mm SAAMI drawing.JPG (87.26 KiB) Viewed 5629 times

You seemed to imply that this is the first time you experienced this problem. If this is an isolated incident, my guess is that shell just happened to be a bad one that either ran at too high a pressure or had some manufacturing defect that made it over-expand. I had the same thing happen awhile back with a heavily-loaded Fiocchi hull. It wedged itself firmly in the chamber after firing, and I tore the rim trying to extract it, just like you show in your picture. I needed to use a cleaning rod to knock it out.

[tom]

Yes, you are right Synchronizor. I checked the actual start of the cone and it starts aprox 10.5cm into the bore (~4.1in), so the shell would not be able to touch that area even after unrolling. And looking at the shell casing now in more detail, it's actually visible that the texts imprinted on the shell have partially come of indicating it has been extremely tight in the chamber from the sides after firing (comparing it to the unfired cases texts).

I'm gonna head out to find me some wood dowels and steel wool and simply polish the crap out of this thing now. Should have done this when you first recommended it for breaking it in

[Kentactic]

Yeah no need to polish the forcing cone. Id wager your issue is at the other end of that shell. Look at the locking lug tab at the back of the chamber. Youll see grooves in it near the chamber. Often there are burrs on that area. Grind/polish out those burrs or deep grooves and you should be good to go.

When the gun is fired the shell is smashed into the bolt face. this expands the back of the shell. The shells drop into the chamber like butter before they are fired. But try and drop a spent shell into the chamber and youll see it binds pretty good on that tab behind the chamber. Any grooves or burrs there will bind the gun up upon extraction.