I have a model 686, 357 mag. It is very difficult to eject spent shells. I was told that if a lot of 38s were shot through it a carbon build up could cause this issue. So I have cleaned the chambers very good. I have attached brushes to a drill and cleaned until it looks like a mirror. Any suggestions?

I would clean each of the extractors on the extractor rod inside and outside. Pay attention to the inside of the extractors as they will accumulate most of the carbon buildup. I would check if the extractor pulls the shells out of the chamber without any hesitation. Stay away from polishing the chambers with a brush on a drill. The recommend approach is with sandpaper with the medium grit to the lowest grit on a drill shaft. I would apply that method if the rounds are not sliding into the chamber. A good cleaning of the chambers with a brush will usually solve that problem. Again, if not, then resort to the sandpaper method.

Nick

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NRA Instructor\GCA Gunsmith

The unfired rounds slide easily into the chamber. I also tried taking the spent rounds and hammering them into the chamber. I was told that is a crude but effective method of removing carbon. But that didn't work either.
How do I attach the sandpaper to the drill shaft?

You have to be very careful with this and have to use very fine sandpaper. I personally use fine steel wool wrapped on an old bronze brush.

Turn Your Drill Into a Spindle Sander Using a Homemade Mandril: 4 Steps (with Pictures)

If the newly cleaned Chambers still exhibit tough extraction then I'd suggest measuring them with a good pair of Calipers. They might not be the correct size and and the only way to know is to use a Caliper.

Try a different mfg ammunition first.
Assuming it's factory ammo,,Might simply be the brass cases of the lot of ammo are a touch softer than need be and are not springing back from the chamber walls after firing.

If it's reloaded ammo, the load may be too hot also along with the above..

If you're going to try sandpaper, I'd be concerned about the grit. I think a better option would be to use a polishing paper. Sorry, but I don't know how to link, but you can Google it. This stuff is like a paper thin green pad, and comes in a pack with multiple grit options. I believe it would be a less aggressive option. Don't have any idea about what's causing the problem with the shells not ejecting. But I have a 327 magnum, that does the same thing, so let us know how it turns out.

What's the history of the gun? Did you shoot hundreds of rounds of .38 and then try some .357? Are both .38 and .357 causing problems? Did you buy the gun new or used? Has this always been a problem?

Is you spray the inside of the cylinder charging holes with a solvent, and then push through a real tight fitting cleaning patch is there carbon on the patch or is it clean?

I would not use sandpaper on the inisde of a cylinder unless it is a last resort and are willing to have the whole cylinder replaced when it turns out worse than when you started.

maybe try a flexhone? Here's the blurb from Brownell's:
Many current revolver manufacturers have been shortcutting their production
processes when reaming cylinder chambers, leaving the internal
finish rougher that was acceptable in years past. The careful use of Cylinder
Flex-Hone®s can correct this situation, and can also go a long way toward
smoothing chambers roughened from light rust pitting. This will allow much
easier extraction of the fired cases - a “must” for duty-type revolvers.

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Joshua Brown(Town)

The flex hone is the best option. When you use it, don't do the full length of the hole. Just go as deep as the chamber and stay out of the throat. I put oil on mine, the same as reaming, or any other cutting operation.

Use an in and out motion, don't just sit static in one place. The problem is usually that the chamber is not a straight cylinder, it has high and low spots along the length that the brass forms into when the gun is fired. The flex hone levels out the high spots and makes the hole a more accurate cylinder shape and smooths the surface, while not removing too much material.

Try a couple of different brands of good factory ammo if the problem still exists send it to S&W before you ruin the cylinder and need a new one.

Once you get the shells out do they have any scrap marks on them?

Do the case walls of the spent cartridge looks straight or is there a bulge in it?

I had one gun that I had to send back to S&W & have the cylinder replaced (under warranty) because the chambers were not cut properly & the fired cases had bulges which made extraction very difficult, no matter what power level ammo was fired.

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657-5 Bulged Cases


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Waiting for the break of day

I would stay away from any sand paper. I would ,and have done myself, use a polishing compound.If you have a Dremmel, with a small buffung wheel, put a small amount of compound on it . If your buffing wheels are to large, spin it over a file until it is a good size to fit the chamber, I have used Mothers Mag polish found at any auto supply .
This is assuming that the chambers are cut properly to size. All you want to do is clean and polish.

Load and fire one round at a time - one in each chamber to determine which chamber/chambers are the problem (mark the problem holes with a pencil).
Look for bulged brass (chamber problem). Flattened primers - firing pin dent raised (pressure problem). Different make of ammo. Unless you are a gunsmith or very talented machinist, and you have cleaned them, you should leave charge hole polishing to others, there's a throat in there thats very easy to ruin. Whatever the final problem, you won't likely need to replace the cylinder unless you damage the throats.
That's my 2.cents, and worth every penny.

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Jeb