642 Bore and Cylinder Measurements, Good?


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Stormin.40
August 12, 2011, 08:03 AM
I had some leading issues with factory 148 gr. WC ammo, this supprised me so I decided to measure my cylinders and the bore to see if anything jumped out at me. I am new to this so I really didn't know what I was looking for.

I took the measurements by tapping a cone shaped fishing weight down each cylinder hole and one down the bore.

All 5 cylinder's measured .3569
The Bore measured .3549, 0.002 less than the cylinder diameter.

These seem to be small to be but what do you guys think, normal? I checked my calibers by measuring some sized .401 bullets I have on hand and it was right on.

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Dogguy
August 13, 2011, 10:19 AM
I saw your post yesterday and figured someone knowledgable would answer concerning the measurements. I don't know if those on your gun are correct or not. I do know that wadcutters are almost always made of very soft lead that can leave deposits in the bore. Most bores are not perfectly smooth so that can compound the leading.

Maybe someone else can answer the measurement question.

Stormin.40
August 13, 2011, 04:39 PM
I thought wadcutters were suppose to be one of the more accurate rounds out of a revolver since there is so much surface area in contact with the bore/rifleing. I may have to do some more internet research to see if these measurements are correct.

I am not concerened about the 0.002 difference between the cyllinder/bore, I think that is about right just thought the cylinder was a little small for lead bullets that are typically sized to .357 or .358.

Notoast
August 13, 2011, 04:45 PM
The cylinder throats should be .357 which is what you got. The bore should not be larger than that which is what you got. I don't think a bore that's 0.002 smaller than the bullet (which will be sized by the cylinder throat) will cause leading. When you measured the bore slug were you sure to measure across two lands (the lands on the slug will correspond to the grooves in the bore), sometimes that's difficult to measure accurately because of the groove pattern.
Also be aware that lead will stick to lead fouling and will also stick to copper fouling. You might try using a bore light and making sure your bore is spotless next time to ensure that's not the cause.

Stormin.40
August 13, 2011, 11:16 PM
Thanks for the reassurance that I have a correctly sized revolver. I will probably throw the leading issue up to cheap ammo and the fact that is is my first revolver, I'm not sure I cleaned it well enough after my first couple of range trips, I purchase a bore light when I first noticed the leading to help me make sure I get it all out for my next range trip. I assume some leading is expected.

I did measure across the two highest point on the slug, and yes it was difficult since they were not directly accross from each other.

Mizar
August 14, 2011, 07:20 AM
If it's a new S&W revolver the barrel is rough inside due to the manufacturing process. Maybe this is the reason for your leading problems. Just shoot couple boxes of jacketed ammunition to smooth the bore - about 200 rounds will do the job.

Boris

Shear_stress
August 14, 2011, 09:28 AM
If it's a new S&W revolver the barrel is rough inside due to the manufacturing process.

It's actually the opposite. If his 642 was made in the last few years its barrel was rifled using EDM, which leaves a pretty smooth finish.

My understanding is that the rounded lands and grooves that result from EDM actually make it more difficult for the barrel to initially grip the bullet--and this may cause leading with some cast bullets.

Mizar
August 14, 2011, 11:15 AM
Well, guess I'm wrong - that is what one gets when he is not interested in new guns.:) I remembered one S&W with EDM barrel and the inside finish looked almost like bead-blasted and I decided that every new Smith is like that... Further reading shows that many people complain about leading in EDM barrels and the general consensus is that the rounded rifling causes it. Bad news for that new 640 I was lusting over...

Boris

Stormin.40
August 14, 2011, 02:22 PM
It is a new 642, does the bore get better after more shooting. Should I shoot a couple of hundred more jacketed rounds? I think I have only put 50 jacketed down the barrel and another 150 lead. Maybe there is more breaking in that needs to be done.

Notoast
August 14, 2011, 06:37 PM
My 642 was bought new this year and has never seen anything but lead bullets. The bore has a glossy polished finish and has never leaded when shooting 148 HBWC or DEWC.
My 686 used to lead when new, but after several applications of J-B bore paste and Kroil that problem has gone away unless I'm driving my handloads at the wrong velocity for the bullet hardness.

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