Slugging an odd numbered bore.

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dpatten
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Slugging an odd numbered bore.

Post by dpatten »

I'm working up a blackpowder cartridge load for an 1868 Trapdoor for dad to take up in the tree stand this year. Testing has determined that it doesn't like the 450 gr. .515 dia bullets which was the nominal bore size for that particular rifle. About half of them keyhole at 50 yards. After slugging the barrel I discovered that it is a 3 groove barrel. So I obviously can't measure directly across the slug to get the measurement.

I vaguely remember a way to do this using a square, but can't for the life of me figure it out. Anybody know of an easy way to do this? I really don't want to have to go ask the guys in the machine shop to borrow their Nikon measurescope...
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HTRN
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Re: Slugging an odd numbered bore.

Post by HTRN »

dpatten wrote:After slugging the barrel I discovered that it is a 3 groove barrel. So I obviously can't measure directly across the slug to get the measurement.
You need a V anvil Mike. CDCO has noname import ones for $80.


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AZMARK
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Re: Slugging an odd numbered bore.

Post by AZMARK »

Here's a page of load information and e-mail address of people who can answer your questions.
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DwightG

Re: Slugging an odd numbered bore.

Post by DwightG »

Can't think of a way to do it with a square but you can measure it with a dial caliper or mic and a 90° vee block if the grooves are wide enough. They probably are on your Springfield. My .45-70 Trapdoor has three grooves that are about 1/4" wide so they cover enough of the circumference to fall onto the flats of a 90° vee block. You can calibrate the block by measuring a known thing like a 1/2" dowel. Measure from the base of the vee block across to the dowel lying in the vee. A bit of trig will go from there. A vee block may be easier to borrow than a Nikon microscope. If you try this method and get stumped on any of it let me know and I'll walk you through it.
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dpatten
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Re: Slugging an odd numbered bore.

Post by dpatten »

Do you remember the trig formula off the top of your head? Getting a v-block shouldn't be a problem.
"Broadly speaking, the short words are the best, and the old words best of all. "
(Winston Churchill)
DwightG

Re: Slugging an odd numbered bore.

Post by DwightG »

Basically the diameter of the slug = dimension÷ (1+√2) *2 where the dimension is to the bottom of the Vee. The dimension for a 1/2"dia dowel comes to .603553 so subtract that from your overall height to get the dimension from the bottom of the vee block to the bottom of the vee. I made a drawing to clarify but don't know if a .pdf will display. The .jpg conversion was pretty muddy but I'll edit that in if the .pdf doesn't show.
VEE BLOCK SLUG.pdf
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Aegis
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Re: Slugging an odd numbered bore.

Post by Aegis »

DwightG wrote:Basically the diameter of the slug = dimension÷ (1+√2) *2 where the dimension is to the bottom of the Vee. The dimension for a 1/2"dia dowel comes to .603553 so subtract that from your overall height to get the dimension from the bottom of the vee block to the bottom of the vee. I made a drawing to clarify but don't know if a .pdf will display. The .jpg conversion was pretty muddy but I'll edit that in if the .pdf doesn't show.
VEE BLOCK SLUG.pdf
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DwightG

Re: Slugging an odd numbered bore.

Post by DwightG »

Aegis wrote:Now my brain itches...
Is that good or bad??? ;)
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JAG2955
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Re: Slugging an odd numbered bore.

Post by JAG2955 »

Couldn't you just measure the diameter many times in many locations, then average all of them? Kinda like an integration of a graph?
DwightG

Re: Slugging an odd numbered bore.

Post by DwightG »

No, because you're always measuring across a land and a groove. Doesn't tell you how deep the grooves are and so doesn't tell you the major diameter.
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