In a recent post of mine it was suggested that I mic my cylinder throats because whatever I fired was going to get sized going through them anyway. Using my digital caliper I came up with .355 on all of the throats on my Model 60. I don't entirely trust my caliper as it might be off by as much as .001 so I tried sliding a couple of different .356 9mm bullets through and they will go when pushed with a small plastic paintbrush handle but there is some noticable resistance. So far all the bullets i've fired have been advertised as .357 diameter. So if my throats really are around .356 would that mean that shooting .358 bullets they were going to get sized down anyway. I've been shooting some Hunter 158gr SWC that are advertised as being .357 and they seem to shoot pretty accurately. I was ging to buy some .358's but it seems like it would only size them down anyway and maybe end up with leading.

I think you have that figured correctly. How is the .357" cast bullet accuracy so far?

If the 9mm bullets are jacketed .356", & you are able to push them through - w/ out a hammer, the throats are probably .3565. Probably just about .001" larger, then bore groove diameter.

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Regards - GCF

The only jacketed bullets I have are Nosler 9mm's and are sized at .355. They go through with no resistance. The cast bullets that are sized at (and mic'ed at) .356 go through with some resistance although they can still be pushed through. The .357 cast bullets really need some persuading to get pushed through. It is actually difficult to say how accurate the .357 cast bullets are because I haven't compared them to anything else yet but so far i am impressed with the little m60's accuracy with them. I only have 135 rounds down the pipe so far but on top of 4.1gr of titegroup it is capable of keeping a 10 shot group within 3.5" with an occasional flyer (operator error probably). This is at a measured 45ft firing offhand. Not sure if this is actually any good for this little 3" pistol but it is better then I had expected and should (hopefully) improve as I gain more experience with it.

Push a .360" soft lead slug through the cylinder and measure the slug with a micrometer.

I would think that if you bullets are getting sized to your cylinder to.356 then your barrel must be that size or smaller. If your barrel is .357 or .358 which is what I would expect for a 357 mag. gun then you would have a problem with accuracy. Slug your barrel and as someone mentioned push a soft lead slug through your cylinder and then measure. But sometimes all cylinder holes might not be exactly the same.

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Tom
NRA Pistol Inst

Well the biggest problem is, I wouldn't even know where to get soft lead slugs of the correct size to slug the barrel or cylinder. I don't cast my own and I'm pretty sure my local True Value doesn't carry them. It would be nice to be able to do it though.

Greased oval fishing sinkers work well. Problem is, getting a good measurement off of a bore slug - as S&W has 5 lands / grooves. You can get close. but it's not precise. 0.355 - 0.356 is what I usually come up with on a SW 38 / 357.

On your chamber throats, figure a .357 (measured) bullet will be a snug slip fit - in a .3575" throat. A slug that is equal size to the throat (.357 / .357), has zero clearance, & will take some force. A .357 lead slug going through a .356 throat, will probably need to be tapped through - w/ a dowel & a hammer.

For the record, most cast bullet manufacturers specify that the bullet should be .001" larger diameter then the groove diameter - when it enters the bore. .0015" - .002" is better.

My $0.02, for what it's worth...

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Regards - GCF

I use Hornady muzzleloader balls. You know they are pure swaged lead. Many fish sinkers are no longer lead due to environmental regulations.

First slug the bore, then take those slugs and check them with the throats. If you can't press them through with your thumb, you are going to have issues due to the throats being undersize. No measurement required at this point.

Couldn't have said it better - leave it to an "over the hill" TEJANO! ;=]

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Regards - GCF

You can't get an accurate inside diameter measurement using digital calipers. You need an adjustable plug gauge and a micrometer, or a micrometer specifically designed to measure curved inside diameters.

Slugging the cylinder is tricky too, because of the short length being measured. It's hard to swage the slug enough to fit tightly and drive it straight through without tipping slightly.

The square edge of the caliper will measure a couple of thousandths low because it doesn't fit the cylindrical surface.

Have a gunsmith check the cylinder throat sizes with plug gauges. They might be different sizes - there were three different sizes on my Ruger GP-100.

Then have a gunsmith or machinist open all the throats to .358" or .3585" and you will be good to go. That step noticeably improved accuracy and reduced leading on my GP-100.