Some Questions--I've got a set of "Minus" Class ZZ Pin Gages so I can measure Cylinder Throats on my revolvers.

I detail cleaned the gun, scrubbed the cylinder with chore boy wrapped bronze brush to ensure all lead and gunk was out.

Then I started slipping gages through the cylinder charge holes. The .358 was no-go, the .357 was no-go, the .356 gage was a "Go".

So, is it fair to say with high confidence that the cylinder throats are .356? And -for best accuracy and least possibility of leading I should use cast lead bullets that measure .356?

What's the difference between Pin gages that are classified as "Minus" vs "Plus"?

I'd use .357" diameter lead bullets. A tiny bit too big is waaay better than a tiny bit too small.

Minus gages are .0002" smaller than the size marked on the gage. Plus gages are .0002" larger than the marked size.

Its all about which way the allowable two ten-thousandths error goes.

A Plus tolerance ZZ gage is +.0002, -.0000, while a minus gage is +.0000, -.0002.

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WWSSD?
What would Skeeter do?

Personally,
If I had one with those dimensions in the throats, I'd spend the $50 at a good gunsmith's to have them opened to what they should be, which is .358". You'll end up with better overall performance with a wider range of bullets without having to speciallty shop for the right size, or have to cast with smaller than standard sizing dies. You'll reduce pressures as well. If your throats are .356", but your bore is .357" or .358", you'll never get the accuracy that the gun is truly capable of.

+1 Ream the throats. .358" if you're going to shoot lead.
Cylindersmith.com Hand reamed cylinder throats

I have reamed a number of guns due to excessively tight throats, but it is usually a good idea to shoot them first. My 627PC has throats I measured with minus pin gages at .357” and it is one of the most accurate guns I have.

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Tom

They could be .358 but just slightly out of round. Pin gages can be misleading that way. A telescoping gage and a vernier micrometer will tell you more.

Pins won't give you an exact size, they will help you determine very closely within a range though. What your findings tell you is that the effective diameter of the throats are larger than .356 but smaller or equal to .357. ( A .357 gage pin will NOT go into a .357 dia hole - the hole would need to be about .0002 or so larger (.3572) at minimum to allow a .357 pin to go. This is from 35 plus years experience in Quality Assurance and Metrology - mostly in close toleranced aerospace applications )

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Sceva
OGCA SWCA NRA

Chamber thoats are only half of the equation... I would slug the bore too.

If your chamber throats are smaller than your bore, you really need to open the throats up. I like mine .0005-.001 over bore size. Then use bullets sized to your throat dia. (or slightly under as noted earlier)

I now have a very nice digital inside mic. The throats of the 627PC measure .35780". I'm happy.

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Tom

Definitely sounds close enough to .358.

Grizzly on-line has PIN GAGE SETS 250 PIECES ON SALE for $63.75 plus shipping.

.251 to .500 undersized .0002, oversize is available along with other sets.

Nice Plastic Case 17 x 12 x 3

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NRA Pistol/Rifle Inst. RSO