measuring cylinder thraots

[bushytail man]

I am using a Starrett telescoping gauge and both digital and regular micrometers.
What is the proper way to do this, insert the gauge from the bullet side of cylinder or the end side?
having mixed readings, but so far I am comfortable with my results of cylinder #
2( .43012 measured 3 times with reg. mics and .430 with digital)
cylinder #3 .43110 measured 3times w/ reg mics. and .431 w/ digital
Clinder # 1 needs to be re-measured, got .43110 then .43010 must have misread. this is time consuming, any help appreciated.
I am measuring a S&W 29. This is first try.
Does anyone know where to get lead slugs for measuring?

[454PB]

I've done a lot of throat measurements using a dial caliper, but slugging is the most accurate and consistent. Using the dial caliper, I just insert it into the throat from the front of the cylinder, then apply light pressure on the thumbwheel while I slowly turn the caliper jaws within the throat. I wouldn't worry about reading five places to the right of the decimal.

I use cast boolit rejects for slugs. They can be reused by placing them in a vice and squeezing them to a larger diameter nose to base.

[Texasflyboy]

Does anyone know where to get lead slugs for measuring?

Wal Mart. Or any sporting goods store. I use Hornady pure lead round balls for blackpowder. Many times you can get a box of leads balls that all you need to do is bump up or down a few thou in order to use as measuring balls for slugging.


Good Page on Hornady Lead Balls (http://www.trackofthewolf.com/(S(ajh2x255k5ybhf23ob4vhgrl))/categories/tableList.aspx?catID=2&subID=25&styleID=58)

[runfiverun]

i do it like 454you just want them as close as you can get them .0001 isn't that big of a deal a thou or more you may want to correct.
then usually .001 over that size will work for you.

[Morgan Astorbilt]

Small hole gages are what you want to use. They're split hollow hemispheres that expand by turning a knurled handle, and then miked with a micrometer or caliper. You just insert, expand, remove and measure. Like telescoping gages, but better for our work.
Morgan

[Bass Ackward]

Bushy,

Is this a new gun? Are there visible swirl marks in the throats from the cutting process? Cause those will open things up .001 as they clean up and wear away.

Main thing I want to see is how the throats were cut and are they uniform. A reamer wears from the front, so you likely get a slight taper on a well used reamer. The basic thing I want is a general sizing indicator and more to know they are all the same. If they aren't, they need to be for best performance.

Different size and dimension throats can make the gun seem like you have misalignment causing a flier(s) or just poor grouping. It manifests itself when you try to size to fit your throats instead of your bore. Or when you shoot high enough pressure loads that you obturate out to what the steel lets it just as if you sized that way. So you form the opinion that hard bullets are more accurate because they resist this dimensional change. Or you decide that sizing closer to bore is more accurate.

So if you decide to fire lap to solve the flier issue and you stop after 48 shots cause you can see the cutting at the muzzle. That was only 8 shots through each throat. Good chance that wasn't enough to solve the problem there. So the performance of the gun doesn't really improve cause the fliers are still there. So now you form the opinion that fire lapping is a waste of time.

A slug is the only way to visualize what's there. A bullet must see the same thing every time. It tells you what battle needs to be fought and the best way to fight it. Otherwise you are taking measurement after measurement and getting something different every time. You start to doubt yourself. That would drive me nuts. But what would be a worse crime would be to NOT do this and always wonder. Or sell a correctable gun.

[cuzinbruce]

The best way I have found to measure cylinder throats is with plug gages. I have tried the most of the other ways and this is the simplest. You don't need to disassemble the gun, which I think you would if you were going to slug it with lead.
I have tried it with the inside blades on a dial caliper, with my Starrett expanding ball gages, with an inside micrometer (Mitutoyo - two little pins). I don't have an Intrimik which might work good but is way expensive.
I have a set of plug gages that came from the flea market for cheap. You want Minus gages. Just a set of steel cylinders about 2 1/2 inches long, ground to size and marked. Graduated by thousandths. Make sure the cylinder and the gage are clean, lightly oiled, then try for size. The largest one that passes is your size. You can check it with a mike. I found my imported cheapos to be accurate as to diameter.