Just interested in knowing what you have found most beneficial in measuring the groove diameter of a slug from a S&W revolver with the 5 lands and grooves pattern.

I have no problem using a 0-1" mic (capable of measuring 0.0001") and successfully measure slugs from a 6 lands and grooves rifling. But the odd layout is challenging to say the least.

You might search for a thread that addressed this a while back. They used shim stock of a known thickness wrapped on the slug,measured then subtracted the thickness x 2.

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KMAG YOYO

You need a special gauge block specific to a five groove barrel. The same applies to any odd number of grooves. There was a machinist several (about 30-35) years ago that made these gauge blocks, but they are un-marked and I do not recall who it was I bought mine from.

Any other method will give you nothing better than a close approximation. Depending on what you are trying to determine. With S&Ws you can get as close an approximation as any method by simply measuring the diameter between the land on one side and groove on the other. For .38-.357 assume a .350 bore, subtract this from what you measured, multiply the difference by 2 and add the .350. So, if your slug measures .354 - .350 = .004 x 2 = .008 + .350 = .358. This should be within +/- .0005".

If you are trying to determine the diameter to size your bullets simply slug the bore, then see if the slug will pass through the cylinder throats with little or no resistance. What you are trying to determine if the cylinder throats are the same size, or smaller that the groove diameter of the barrel. If the slug passes the throats then slug the throats and size your bullets this diameter to that diameter to +.001".

If the bore slug will not pass through the throats then you have to decide if you will live with it, or will enlarge the throats. If you choose to enlarge the throats see above. If you choose to live with the condition, then see above! The best you can do is size throat +.001 either way. Forget sizing to groove diameter, you need to fit the bullets to the throats for best accuracy.

Note, in determining what diameter to size your bullets, nowhere is it necessary to know the groove diameter of the barrel! All that is necessary is to know how the bullets fit the throats.

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Gunsmithing since 1961

I think the v-block mentioned above was cut at a 105 degree angle. If so you should be able to have one made by any machinist friend who is short a 6 pack at the moment.

This issue of Handgunner magazine has the information on V blocks.

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JIM
NRA Pistol Instr

For about $63.75+shipping (IT'S ON SALE NOW) you can purchase a set (250 pieces) of sized Ground GAGE Pins.

My set is .251 to .500, 250 pins, .0002 minus (undersized). I use to measure the cylinder bores. The size of the pins are marked on them.

The set comes in a plastic carrying case 17x12x3 inches. A NICE set.

Pin set is from GRIZZLEY.Com

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NRA Pistol/Rifle Inst. RSO

That is a nice price - especially considering individually they go for about $6 each.

Thanks Cochise, that was a good tip as I didn't know each V block has its own adjustment factor. I suppose it is something that can be calculated from the block/V dimensions if you stare at it long enough.

I very much appreciate the responses.

I do have a set of pin gages. They can be useful, and I realize their limitations (provide you with the minor diameter, not the major diameter).

I also understand it isn't absolutely necessary to know the groove diameter. It is useful information to help guide your actions however.

Case 1. I had 3 Ruger cylinders to deal with (2 were 32 H&R Mag, one was 32-20). On all 3 a .312 jacketed bullet wouldn't pass through any of the throats. Using pin gauges a .311" would pass. I checked the grooves on both barrels and they were .313" The Manson reamer I got was to open the throats up to .3135". So I felt I was making the correct action to use that reamer with the commercial cast bullets (.313" diameter) I wanted to use. So at least I knew everything would be want I wanted. It worked out just fine.

Case 2. I have a M14-2 S&W. I am shooting .358" DEWC bullets. They will not pass through the throats. Using pin gages, the throats measure .357". A slug from the barrel passes easily through the throats. I am considering moving to .357" DEWC bullets, but I just would like to know what the groove diameter is before changing. Since the bullets pass through the throats so easily, I wanted to try and measure.

So in either case knowing the groove diameter is just one added data point to help in making a decision.

Thanks for the info. I just ordered a set - $63.25 + 11.95 shipping. They also have a lot of other useful stuff at good prices, like steel rulers with 1/64 inch graduations. The name of the company is GRIZZLY.com, there's no E.

jepp2,

You say,

The gage pins may give you minor diameter AND NOT THE MAJOR.

In a revolver the Diameter of the Cylinder Bores or Plus .001 is all you'll usually need.

When the bullet passes through the cylinder into the barrel, THE CYLINDER BORE SIZE RE-SIZES THE BULLET TO THE TRUE CYLINDER SIZE.

If the MAJOR BARREL diameter is larger than that, you're screwed, unless you have the cylinder bores opened up by a gunsmith, and then try again.

I have a model 29-5 44 mag with all six cylinder bores .433, but I'm assuming S&W barrel size is around .430, I'll size lead bullets to .433 and .434 to try accuracy tests.