Are 686 cylinder bores stepped?

[Sawdust]

I'm cleaning the cylinder of a new S&W 686.

Are the bores of the cylinder stepped? Or are they the same diameter all of the way through?

I ask because when I put a wet patch on a jag, I cannot push it all of the way through.

If the cylinder bores aren't machined with a slight step, then I guess that I have some tough fouling rings from shooting .38 specials.

Thanks,

Sawdust

 

[Jim Watson]

The cylinder is reamed with a chamber and a throat. I guess you could call that "stepped." Have you tried a brush, solvent, and elbow grease?

Somebody suggested using a fired .357 case, not resized, case mouth flared as much as would go back in the chambers, as a scraper to remove fouling from the .38 Special casemouth position.

 

[R.H. Lee]

http://www.thehighroad.org/showthread.php?s=&threadid=98010

 

[Sawdust]

The cylinder is reamed with a chamber and a throat. I guess you could call that "stepped."

If I interpret your answer correctly, the chamber is machined to a certain diameter, then the throat is machined to a different diameter (actually vice versa), and there is a sudden transition betwix the two...yes, that would be "stepped".

Do you know what the difference is between the two diameters?

RileyMc:

Yeah, I'm hip to that thread; it discusses how to clean the fouling, not if the bores are machined with a step. I will use that info during cleaning...good thread.

I asked my question so that I know when I'm done cleaning; i.e., will I always feel a step, or not.

Thanks,

Sawdust

 

[R.H. Lee]

I asked my question so that I know when I'm done cleaning; i.e., will I always feel a step, or not.

That's a question I'd like answered, too. I can't figure out if it is "stepped" or if the ridge inside the chamber was caused by hot gas erosion.

 

[Black Snowman]

Tapered or profiled might be a better term. I can't get to Brownell's site from work but if you can find a picture of the chamber reamer it will show you what the profile looks like.

There will always be a transistion and a tapered "step". In a few revolvers that headspace from the case mouth, like a 610 or 645 that use "rimless" cartridges, you'll get an abrupt step for the case to stop against. Rimmed cartridges that don't have a shorter counterpart that can be fired in the gun (like some .38 Specials) may also have this abrupt step.

If there is no step that's something to be concerened about because it means either the cylinder is too short for the round it's chambered in for proper construction or it's been re-reamed to a longer cartridge than it was designed for.

For getting stubern crud out of those profiles without damaging them I like the coiled wire "Tornado" brushes.

 

[Jim Watson]

Chambers are reamed. I THINK the factories still drill the cylinder at each chamber location, ream the throat diameter at or slightly above barrel groove diameter, then ream the chamber. Probably have a big CNC machine that does it FAST.

SAAMI .357 cylinder throat specification is .3580."
The chamber proper diameter specification is .3800" at the casemouth, .3809" near the case head.
Tolerance is plus up to .0040", minus nothing; but you should certainly hope for a major maker to hold it closer than that.

There is not a sharp step from chamber to throat, it is just shadows that can make it look that way. The transition is a taper over about 0.1"

An elevated "ridge" would have to be accumulated fouling. It should scrape or brush out. Erosion would be a groove or pits. I have shot many thousand .38s through my Python and it shows no damage to the chamber when thoroughly cleaned.

 

[Sawdust]

Here's the reamer mentioned by Black Snowman:

Looking at it (starting from the right end and going to the left) from the tip, it looks like it tapers from small to large. Then it appears to step-up suddenly in diameter for a short distance (can't tell if it is tapered or straight for a counterbore in that last part).

So, from the looks of that reamer, it appears that the bore cylinders are a straight taper.

That means that one should not feel any step if the bores are clean.

Agreed?

Sawdust

 

[Sawdust]

OK, Jim, you got that one in as I was posting my last message.

I understand what you are saying, and it makes sense. I'm not a gunsmith (obviously), but as an engineer it would make sense to me that the cylinder would be tapered all of the way through with the throat (the end closest to the barrel...right?) the small diameter of the diameter of the taper. Then the bore would be counterbored (constant diameter) from the back end (end closest to the trigger) almost the full length (i.e., a short taper would be left at the throat end).

Hmmmm...however, that reamer picture sure doesn't look right then.

Bottom line is that it appears that no step should be evident; if so, it's from fouling. I need to get a bigger brush, I think, as the only one that i have is for my 9mm pistol.

Sawdust

 

[Jim Watson]

Regardless of what makes sense, or what Sawdust says, revolver chambers are not cut on a continuous taper at any stage I know of. I was reading from the SAAMI official print.

The picture does not look like a .357 magnum reamer to me, more like a bottleneck rifle chamber.

A "step" IS evident, I just looked through the cylinder of a .357 revolver that had been fired a few .38s since cleaning. The "step," actually a short taper from chamber diameter to throat diameter, as previously described, is very clearly visible, as is a band of fouling from there back to the location of the .38 case mouth. It is easily felt with a patch or brush. If you can't get a patch through, your jag is the wrong caliber or your patch is too big or too thick.

The deleading reamer shown at
http://www.brownells.com/aspx/NS/store/ProductDetail.aspx?p=5040
is made like a chamber reamer, just undersize to scrape out really, really severe lead fouling.

 

[Jim K]

Most modern revolver cartridges (except the .22 LR) are what is called "inside lubricated". In terms of dimensions, that means that the bullet fits down inside the case. Obviously, then, the bullet has to be smaller in diameter than the outside of the case.

The rear part of a revolver chamber is of a size so the outside of the case will fit. But to keep the bullet from swelling up under pressure and distorting as it enters the barrel, the front part of the chamber is made the same diameter as the bullet. That way, the bullet has a better transition from the case to the barrel without excessive bulging or distortion.

That is the reason for the "step" in normal revolver chambers. Revolvers made to fire rimless automatic pistol cartridges, like .45 ACP or 9mm Parabellum have a sharp shoulder so the case mouth will act as case support, but that is another issue.

Edited to add: The "step" also serves to prevent chambering a round longer than that for which the revolver is intended, such as a .357 in a .38 Special.

HTH

Jim

 

[Sawdust]

Mr. Watson:

Allow me to help you understand that what I said in no way contradicts your interpretation of your "SAAMI print".

I, in fact, stated in my post that what you said makes sense to me. Obviously you have no machining experience, for what I next iterated in my post was the machining sequence that would result in the geometry that you described.

My post:

"I understand what you are saying, and it makes sense. I'm not a gunsmith (obviously), but as an engineer it would make sense to me that the cylinder would be tapered all of the way through with the throat (the end closest to the barrel...right?) the small diameter of the diameter of the taper. Then the bore would be counterbored (constant diameter) from the back end (end closest to the trigger) almost the full length (i.e., a short taper would be left at the throat end)."


Interpreted short and sweet for you:

If you cut a tapered through hole, then counterbore it deep, you end-up with a hole with constant diameter over the majority of the length, and a small taper at the end.

Oh, and the reamer that I posted is straight from the Brownell catalog next to the entry "Pistol Chambering Reamers";you can check yourself:

Pistol Chambering Reamers

If that isn't your idea of a chamber reamer, well gee, didn't I agree with that, too? But you can take it up with Brownell's yourself.

HTH,

Sawdust

 

[Jim Watson]

Right, I'm not a machinist, just a chemcal engineer who has to call one when metal work is needed. Did not mean to offend.

But I still don't think a revolver chamber is first cut on a taper. The chamber reamer pilots off the smooth straight hole drilled/reamed at throat diameter.

I guess you could say a rifle chamber was cut from a taper since the usual procedure is to first run in a roughing reamer that cuts about .010" under finished size to save wear on the finishing reamer.

Upon reexamination, the illustrated reamer probably IS a pistol reamer. Thing is, the working part that actually cuts the chamber is about 1/4 of the length. Most of the long fluted section shown is just unused blank.
I have a 9mm reamer borrowed for a project, so many factory chambers are undersize due to advertising a "minimum match chamber" or trying to squeeze a few more guns out of a worn and resharpened reamer.
It has a .250" pilot, about .010" of throat or leed for an auto, and .754" of chamber. But it is about four inches long from the tip to the end of the flutes. Either for inventory convenience of a few standard reamer blanks, or to give reach in the setup, or the setup of their flute grinding machine; I don't know the details, but there is a lot of non-cutting length there.

 

[BluesBear]

The reamer pictured is for a bottlenecked cartridge such as a .30-06.
It is NOT a .357 reamer! Not even close. I don't care where thay placed the image in the catalog.


From back to front, a .357 magnum cylinder will be bored to a diameter of approximately .381" for a depth of 1.30" and then will be bored the remainder of the way through with a diameter of approximately .359".

Rimmed revolver cartridges, since they headspace on the rim rather than the mouth usually use a short tapered area to transition from the larger diameter of the actual chamber area to the smaller diameter of the cylinder throat.

Rimless or semi-rimmed cartridges that headspace on the case mouth have a distinct "step" where the chamber meets the throat. This will be readily apparent whan you look into the cylinder of a revolver chambered in .45acp or the barrel of almost any semi-automatic pistol.

If you can find a Ruger Blackhawk convertable in either .357/9mm or .45 Colt/.45acp you'll see the difference when comparing the cylinders.