Hi. I have a M69 and M629-5; throats on both measure .428” using pin gauges. I don’t reload and accuracy with factory ammo is okay but not great. Haven’t figured out how to slug/ measure bore diameter because both have 5 grooves, and it would seem a mic would be measuring between one land and one groove. I assume my bores are .429”, lacking better information. Two questions: how do the pros measure bore diameter given an odd number of grooves. And would I gain anything by having the throats opened a bit (again shooting factory ammo). Thanks!

It would surprise me if the throats are smaller than the bore on 2 guns.
When I slugged my 69 and 629-5, and mic'd the slugs I got throat diameter 4295-430.
Even with 5 groove rifling, if you slug the barrel you can catch the edges of 2 grooves 180 degrees apart. I got 429 when I did it that way.

Both my 629s have cylinder throats that measure .4285 with plug gauges. I load Keith style lead bullets sized .430. Accuracy is excellent. If you open up your cylinder throats you will open up your groups.

A .428” pin gauge passes throats of both Smiths; a .429” does not, so .4285”?. What got me stewing about this is my Blackhawk has .431” throats and it seems to shoot a bit better, but then it is 7 1/2” vs 4” and 5” for the Smiths. Dunno - maybe the smoke here is addling my brain..

There are many factors involved in this equation. Forcing cone, muzzle crown, barrel constriction (or lack of). For best accuracy, all these need to be to spec. Often, one or more is not.

Normally the throats should be bore diam. or +.001. Bullets to fit the throat. The barrel can easily deal with a bullet a thou. or 2 oversize, even jacketed.

My M69 snubby has .4290" throats but my M396NG had .4285" throats. I purchased a Cylinder Throating Reamer with a removable pilot from Dave Manson Reamers when I bought the a 10mm Finishing Reamer for my 1013 Project. It opened the throats up to .4307" which worked well for me since I use .4302" jacketed bullets in it.

It's the groove, not the bore, that you're looking to find it's size (~.429"). If you already have a pin gage set you can easily find it's bore size by seeing what's the largest one that will freely slide down the barrel.

After I find the bore size (ex: .4168"= "A") I use a telescoping gage to measure from one land (5 groove rifling) to it's opposite groove, then measure that distance (ex: .4232" = "B") with a micrometer (not caliper).

Subtract the second reading from the first ("B" - "A"="C") & you have the depth of the lands. Add that to the second reading ("B" + "C"="D") &, if you held your breath just right, you should be in the ballpack for it's groove diameter (ex: .4232" + .0064" = .4296") which is what I got for my M69.

It's a few measurements but I never seem to be able to find, or have, the lead that I need when I want it (which is occasionally) so this works for me. And the lead sizing kits seem overpriced, plus to correctly measure the slug afterwards you need a special form to put it in for that.

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Throating Reamer


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Pin Gage set


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Waiting for the break of day

To answer one of the OP's questions, the bullet pro's would use a V-block micrometer to measure S&W barrel slugs. There are many versions, here is one from Mitutoyo. V-Anvil Micrometers - Series 314, 114 - 3 Flutes and 5 Flutes

Stu

Notice that the 3 flute v anvil mike has a much more open angle (60 degrees) than the 5 flute mike (108 degrees). If you invest in one of these, make sure you get the right one - 5 flute. Check out CDCO Machinery Corp. for better prices.

Thank you everyone for your knowledgeable replies. I gotta admit investing in a v anvil mic is probably a deeper dive than I care to make, although it is interesting to learn they exist. It also sounds like throats are about where they should be; certainly sufficient for “minute of deer” accuracy.

I think 1 thou. undersize is not something to lose any sleep over.