Question on measuring bore diameter/slugging the bore

Measure the "high spots", bullets that are .001" larger or a bit more are a good starting place. There is really more to it than that.
Lafitte

Depending on number of groves/lands you may not get a good accurate reading if odd number. Takes a special V base to support across the lands. On revolver you need to measure the cylinders (all) for they may not all be the same size. If your cylinders are smaller than your bore you have the perfect setup for LEADING. Which is not a good thing since they will size down the bullet before it hits the forcing cone.

Throat diameter, bore condition, lead alloy hardness, and caliber can all play a part in finding the "correct" diameter for lead bullets.
Lafitte

usmc0811 said:

Wats a V base?

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Do to the land/grove spacing you will contact in between them, you can get close. To get a accurate reading you need the right tool.

https://www.hroberts-di.com/micrometers-c11/mitutoyo-114-121-5-flute-v-anvil-micrometer-5-25mm-p733

If your shooting lead in a semi-auto you might as well just go with with the largest diameter that'll chamber.

For a revolver just measure the cylinder throats. You want something that pushes through with a little pressure or something slightly larger than that.

You can skip sizing bullets if you want to take a halfway approach to it... If you're going to take the time to cast bullets and reload your own ammo you might as well do it right and take a couple of minutes to size the bullets. It's the easiest part of the process and essential if you want anything more than mediocre reloads.

Just a note about bore slugs, very easy to make by smacking a lead bullet of the correct caliber on the nose with a hammer. You will find it expands more than enough to get a bore "print" with full contact, I would advise an aluminum rod too. a wood dowel can split and wedge thing up. don't forget to oil the bore to make it easier.

One way to slug your barrel:

• Use a vise/hammer to squeeze a lead RN bullet to slightly larger diameter than barrel's bore

• Tap the bullet partially into the barrel

• Tap out the bullet with brass/aluminum rod (Or 1/4" socket extension wrapped with paper towel/plastic to protect the barrel/rifling)

• Measure high spot-to-high spot for groove-to-groove diameter of the barrel

usmc0811 said:

measure the parts on the slug that are HIGH spots that would be the groves of the bore,

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Do this.

When there is an odd number of grooves like a S&W, the high spots become harder to measure.

Needed -a micrometer, a strip cut from a soda can. Vice grips.
Measure the thickness of the aluminum can strip. Should be around .004" Write it down.
Wrap strip around bullet tightly and lock it on with the vice grips. Measure, subtract X 2 thickness of strip. This is your groove diameter. Example- .460" - .008" = .452"

I size my cast bullets for handguns at groove diameter or a hair larger. The standards seem to work for me with the correct alloy & Lube.

Standards for cast .3575/.358" & .430" & .452"

I size/lube using a Lyman 450. If the diameter is ok as bullets drop from the mold, coat with Lee Alox, load and shoot.
A larger diameter is ok if rounds chamber freely.

Hate to say bbbbuuuuuuuutttt, been at this a little while and I've never slugged a bbl.

Revolvers:
It's all about the size of the holes of the cylinders. You want all holes to be even and a cast bullet to push thru them with minimal effort. After that read the bbl for leading and where the leading is. For all bullets read the target and that starts with all bullet holes being round.
semi-auto pistols:
Do your 1st outing with factory ammo checking for function. Measure the od of those cases and subtract the wall thickness from the od (brass thickness 2x). This will tell you the largest bullet you can use in your chamber. I typically use a bullet 1/1000th's under that max # from the od measurement. Again your bbl and target well tell you if it's wrong or right.
Rifle bbl.'s:
It doesn't do any good to run bullets bigger than the ball throat, I size everything cast/swaged to the ball throat diameter. Again your bbl and target will tell you everything you need to know.

I do these things 1st and then if there's issues I'll did deeper into that firearm.

The "high spots" on the slug are the groove diameter of the barrel. That is about the minimum for bullet diameter in most but not all cases. To measure the actual bore diameter (not the Internet Bore Diameter) you would have to have a blade micrometer to get down in the grooves on the slug which were made by the lands of the barrel.