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Old January 7, 2013, 10:32 PM   #1
yggorf
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Polishing rifle chamber

Using a borescope, I have detected some scratches on the chamber walls and neck area of a new rifle ( never fired). These scratches are just barely visible with the naked eye, but with the magnified bores cope they are obvious. Chambering a case and extracting it the scratches don't transfer to the case -- but they might under normal firing pressures.

I'm wondering if I can lightly polish these out using 600 or higher grit sandpaper without messing up the chamber dimensions. This is a 260 rem if it matters.

If so, how would you go about it?
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Old January 7, 2013, 11:14 PM   #2
James K
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Do not try to polish the chamber until you fire the gun and see if there is any problem. If there is not, put away the emery cloth or sandpaper(!!!). You can easily mess up a chamber by trying to polish it if it doesn't need it.

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Old January 7, 2013, 11:35 PM   #3
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First, follow James Ks recommendation, then, if you must... Flex-Hone makes chamber hones in 400 and 800 grit for specific cartridges. They are intended to "finish" a cut chamber with a smooth, specific angle crosshatch... typically 45 to 60 degrees.

I would not suggest using these without a thorough understanding of what will happen if you use a Flex-Hone incorrectly... they can be, in the wrong hands, very "aggressive". A pretty decent thread on the subject of chamber polishing can be found here.

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Old January 8, 2013, 12:32 AM   #4
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If the gun shoots fine and the brass is undanaged, leave it alone.

We made polishing sticks out of 01 round stock. After filing one end flat (held against a machinust square), We brazed a brass dome on it. We then sawed a one inch slot down the center. Voila, polishing stick. We turned our barrels on the lathe and inserted the stick with a 1" emery strip wrapped around it.
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Old January 8, 2013, 12:04 PM   #5
yggorf
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OK

OK, I'll leave it alone unless a problem develops. I was a bit worried there may be extraction problems with a non-smooth chamber wall.

By the way, Creeper, where do I find those cartridge specific Flex hones? Brownells has just straight hone of various diameters -- is that what you were talking about? Which one would be appropriate for 260?

Last edited by yggorf; January 8, 2013 at 12:09 PM.
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Old January 8, 2013, 03:36 PM   #6
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By the way, Creeper, where do I find those cartridge specific Flex hones?
They are available from Midway USA and direct from BRM (Flex-Hone).
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Old January 8, 2013, 03:39 PM   #7
Brian Pfleuger
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I certainly wouldn't use sandpaper to polish a chamber but I have polished the chambers on all my guns, using polishing compound and a bore mop. No idea if it helps anything but I see no evidence that it's hurt.
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Old January 8, 2013, 04:25 PM   #8
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Quote:
No idea if it helps anything but I see no evidence that it's hurt.
Interesting Brian. Chances are that it would make a otherwise standard production chamber a little easier to clean, with no negative side effects that come to mind.

I've seen production chambers that are loaded with micro chatter marks and tooling scars when viewed with under bright light and a magnifying glass... and I've seen benchrest chambers that are lap finished to 0.4 ~ 0.1 Ra (μm), and when viewed with a 24X borescope look like a very fine grit, mirror finish polishing job. Blackstar finished barrels are even smoother than that. Take a look at the before and after.

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Old January 8, 2013, 06:21 PM   #9
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Part of my moldmaking career included being sent to Chicago to understudy a master mold and die finisher.He was working on a coining die for the mint when I showed up.

Regarding the OP's scratch.A surface finish has high spots,low spots,and uniformity.A chamber reamer leaves a tool finish with the lines running perpendicular(pretty much) with the bore axis.To be able to see a flaw in the PATTERN,the lines,may indicate the tops of the texture have been disturbed,This can be visible with practically no dimensional disturbance.If the flaw is very narrow,less than the case wall thickness,it is unlikely the case will flow into it at reasonable pressures.Think in terms of a 1903 Springfield with a cone breech or a handgun with unsupported chamber.

At least now your chamber is round,form and dimension correctness.Polish the low spot out of your scratch,you will likely compromise form and dimension. When I polish,I carefully polish off the high spots till all the low spots disappear at the same time.Then the low spots define form and dimension.

Now,as your tool marks ordinarily are perpendicular to the bore for direction,if you use a mop or soft media with abrasive,as it cuts the tops off the tool finish,it also conforms to the low spots on the tool finish and cuts them deeper!

You must stroke in the direction of the bore across the lines of the tool finish.

You want the stone or lap to bridge over the low spots ,and not cut them.You only want to cut the high spots

The first,essential step to a good polish is a good tool finish.Cut abad chamber with a dull reamer,or straddle a chip over a cutting edge,you aren't going to fix it polishing.

I ordinarily do not polish a chamber,but,if I did,I would run the lathe as slow as it will go in back gear,dress a 600 or 800 grit soft mold polishing stone ,use kerosene,and uniformly stroke the length of the chamber as it rotates.

This will make a crosshatch.Inspect frequently,only cut 15 seconds or so at a time.As soon as you have a uniform change from cutter marks to crosshatch,stop.

I would do nothing more,other than the radius corner beak at the chamber mouth.

Last edited by HiBC; January 8, 2013 at 06:34 PM.
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Old January 8, 2013, 06:50 PM   #10
yggorf
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Interesting...

It would be good to know if there is a "standard" for chamber wall smoothness (either measurable like "no scratch/low spot greater than .0005", or subjective such as "not visible under 5x magnification").

My scratches are much less than the thickness of a Lapua brass case wall - probably one tenth of that.

I'd also like to understand better the correlation between these kinds of chamber surface imperfections and accuracy. Do they matter at all?

I'm guessing no - but the next question is "what about feeding/extraction reliability"?
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Old January 8, 2013, 08:11 PM   #11
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How about this: If you're seeing scratches in the chamber of a new rifle, how about sending it back to the maker for correction.
Let the people who built it decide if it needs polishing or replacement.

Light scratches have no effect on accuracy, and the only way to tell if they'll cause extraction problems is to shoot it.
If they do, contact the maker and have them send you a shipping label so you can ship it back on their dime.

If you start polishing, you'll void the warranty and you'll pay for repairs.
When you pay good money to buy something new and it's defective, you don't try to fix it yourself.
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Old January 8, 2013, 08:14 PM   #12
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Guys, be honest. Is there any really logical reason to polish your rifle chamber, or is it that you've just run out of places to polish or stuff to do? I used to have a Son-in-Law (since replaced) that used to polish the bottoms of his shoes. To me, that's in line with polishing your rifle chamber. If you have an old Mosin, that's one thing, but any sort of new rifle shouldn't need chamber polishing.

Hmmm, that brings up another question. How many of you guys put your resizer case holders in your tumblers to get em all shined up? I honestly hope that nobody admits to that. And then there's a couple of you fellows that never thought of doing that - but probably will do it tomorrow. What did we call OCD before it was OCD?
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Old January 8, 2013, 08:30 PM   #13
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yggorf, your fired cases will shrink enough from the chamber walls that those rough marks won't hurt a thing. Besides, they help hold the case in place and reduce back thrust on the bolt face. Chamber walls too smooth is not a good thing.
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Old January 8, 2013, 10:22 PM   #14
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You can actually polish a chamber and CAUSE stuck cases. If you polish the chamber to the point that the front part is larger than the back the case will swell into the larger part and won't extract. That is why I recommend not polishing anything unless 1) it needs it and 2) you know how to do the job.

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Old January 8, 2013, 10:38 PM   #15
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You'd have to do a lot more than "polishing" to cause problems. Polishing compound like Flitz isn't going to remove more than .0001".

In reference to polishing mild steel, VarmintAl says "There is no detectable decrease in the diameter on either end measured with my high-precision Mitutoyo Micrometer." after 2 minutes polishing at 500 rpm using Flitz...

http://www.varmintal.com/arelo.htm
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Old January 9, 2013, 01:29 AM   #16
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I often wish people would just polish chambers with Flitz. I have seen chambers that look like they were polished with 100 grit paper, bright but rough as a corn cob. And the guy complained about hard extraction . . .
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Old January 9, 2013, 03:01 PM   #17
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Brian, meet Scorch. His experience has been mine also. When someone says "polish the chamber" he may mean Flitz, or 40 grit paper. I have seen chambers so "polished" that the condition did occur or I wouldn't have mentioned it.

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Old January 9, 2013, 03:11 PM   #18
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If it's a brand new rifle, since you indicated that it's never been fired, contact the manufacturer and put them on notice about what you've discovered. Inform them that you're going to go out and fire the rifle to see if there are any issues. If there are no issues with the cases extracting or anything else, contact the manufacturer and let them know that it's all OK. If there are issues, then they will have the opportunity to make it right.
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Old January 9, 2013, 03:46 PM   #19
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I wouldn't do anything more than a good scrubbing before shooting the rifle. If your brass extracts well and comes out without blemishes it's fine. Chambers generally aren't highly polished.
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Old January 9, 2013, 05:22 PM   #20
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Anything faster cutting that crocus cloth needs very careful consideration before you apply it, and even then it must be judiciously. The main advantage to highly polished chambers is longer brass life for those doing full length resizing. Because the case sticks less well to the smoother surface, it allows the stretch in the pressure ring to be distributed over a greater length of the case wall, with the result that it doesn't thin out as quickly. If you use a sharp reamer and through-bore pressurized cutting lubrication, you can get it very smooth and pretty scratch-free.

Flitz and other polishes have been shown to work partly by microscopically smearing the surface to fill irregularities rather than simply removing high spots (though they do do some of that, too). But it's likely removing and leveling the surface on the order of two to five millionths of an inch or so (2 millionths or less ripple is a typical mirror spec). It takes awhile to remove much material working on that scale. So Flitz is pretty safe to work with. It won't remove a big scratch, but can make its edges less sharp so it ceases to mark brass.
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Old January 9, 2013, 06:36 PM   #21
Brian Pfleuger
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Quote:
Originally Posted by James K
Brian, meet Scorch. His experience has been mine also. When someone says "polish the chamber" he may mean Flitz, or 40 grit paper. I have seen chambers so "polished" that the condition did occur or I wouldn't have mentioned it.
Not saying it doesn't happen, I'm saying that it's not "polishing". That's like claiming that taking a submarine to Challenger Deep is "going swimming".
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Old January 11, 2013, 04:30 PM   #22
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I agree that, unless fired brass is deformed, leave the chamber alone or send the rifle back to the factory for service.

If brass is slightly deformed by the scratches, I'd take a fired case, modify the base so it will take a screwdriver shaft chucked in a drill, coat the case with Flitz or JB Bore Paste, then rotate it slowly in the chamber with light force. Such polishing shouldn't be so aggressive as to change chamber dimensions.

In an older rifle whose chamber has rusted, a brass brush with 000 steel wool on a cleaning rod, turned by a drill works well. Too little polishing is better than too much!
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Old January 11, 2013, 08:30 PM   #23
yggorf
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I'm not polishing!

Don't worry, I'm not polishing (until I get a chance to shoot the rifle and see if extraction works).

But, I am curious -- where does Flitz fall on the grit scale --400, 600, 800, ...?
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Old January 12, 2013, 01:55 AM   #24
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Flitz is extremely fine, finer than rouge or gypsum. It contains talc. So does baby powder, just to give you an idea.
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Old January 12, 2013, 02:34 PM   #25
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I don't think there's any "grit" rating for Flitz. It says "non-abrasive" on the bottle. All it does is turn a smooth finish into a mirror finish.
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