I recently read a post in another thread describing the common practice of matching cast bullet diameter to chamber throat diameter. I was surprised to see the chamber throat dimensions for a Model 16-4 listed as .311".

Why surprised? Just a couple of days ago, I had measured the chamber throats and chambers in two different Model 31-1's, a Model 16-4, and a Model 631. All four showed the exact same dimensions for chambers and chamber throats; 0.341" for the chambers (Mic'ed at 0.34085") and 0.314" for the chamber throats (Mic'ed at 0.31375"). Initial measurements were obtained with pin gauges, the gauges then checked with a digital micrometer.

So, do I have four oddball dimensioned 32's? Has anyone else ever checked these dimensions? Please share your findings.

Thank you.

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Dave Brown
SWCA #3279

Pre regulation police 32 Long built about 1952 0.311".
Ruger single six mag 32 H&R 0.3125"
Charter Arms Professional 0.315"
I have shot a friends but never seen a Model 16 for sale in person.

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Front sight and squeeze

Some throat dimensions from guns I have owned over the years. Down to 3 now, the 31-1, K32, and 32 M&P.


SP101 327 Magnum .3125

S&W pre-war 32 Long M&P .3125

S&W K32 32 H&R conversion .3135

S&W 31-1 .314

S&W 631 .314

S&W 16-4 .314

Uberti Stallion .314

Bisley 32 H&R .3145 (New out of the box 2 throats were .314 and the other 4 were just over .313. -- .3145 was the final dimension of all 6 throats after having them honed to a uniform size.)

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S&WCA #1955, S&WHF #818

Thank you for sharing your findings. It looks like my four examples are within the expected range. They all shoot great, so I guess everything is good to go!

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Dave Brown
SWCA #3279

The "book" says what should shoot well but mister target shows the final verdict.

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Front sight and squeeze

Technically speaking:

I always slug the barrel to find the groove diameter.
I pin gauge to find the chamber exit diameter.

I want my exit diameter to be from .001 to .002 over groove diameter for the bullet to obterate and seal the grooves from hot blow-by. I also want my bullet diameter to be the same as my exit diameter at from .001 to .002.

A nice fit would be a .313 bullet, a.313 exit hole, with a .311 groove diameter.
or a .314 bullet, a .314 exit, with a .312 groove.

You need the bullet diameter to be the same as the exit hole. Too large and they will be swaged down when they hit the exit. Too small and they tend to rattle down the chamber from side to side.

Of course, if you shoot lots of different bullets and get good accuracy; it's not broke, and don't fix it.

Technical stuff is required when the accuracy sucks or you're leading the barrel. It gives you a place to start to try and find fixes.

Prescut

I'm not familiar with the .32 magnum, but have had a couple of late model ('70s) S&W .32 Long revolvers. I treat them like revolvers in other chamberings. I cast some bullets and size to different diameters and shoot some twenty-five yard benchrested groups. This method will quickly show the best bullet diameter for a particular revolver.

Slug the bore if you wish, but it's seldom necessary. For my two .32 revolvers, bullets of .314" were the best diameter. I suspect this diameter will work for many S&W revolvers, but not all.

My 16-4, has cylinder throat diameters of .311, and grove diameter of .309, as the best I can measure 5 groove rifling. I thought I had an oddity here, but apparently not. I had been sizing my bullets @ .311, with no leading.

In my case, attempting to have all my bases covered so I might be able to purchase a chamber finishing reamer to convert my two Model 31-1's to .32 Magnum if the people at Dave Manson Precision Reamers ever decide to answer the phone.

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Dave Brown
SWCA #3279

You can rentt one from 4DReamer. I did.

I have never measured any of my I and J frame 32s nor my 16-4. I guess I should. I have 2 K frame 327s made from K22 cylinders that have .312 throats.

By the way a 5/16 chucking reamer is .3125