Bore diameter vs. groove diameter from a pound casting

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harleyrock posted this 18 February 2019

I am planning to attempt paper patching the Lee 7mm soup can bullet.  In reading about paper patching, writers on the "other" CB forum have advised " do a pound cast  and make your boolit .001" to .0015" over BORE diameter" (their emphasis, not mine).  

I have done "pound casting" of my throat and the first ½ inch of the barrel on all of my cast bullet guns, but I don't know how to measure the "bore" of the gun from that. I can measure the bottom of the grooves easily enough with my micrometer, but I don't know how to get it down into bottom of the grooves of the casting which is where the bore diameter is. The casting is a reverse of the barrel in that groove dimension stands out whereas in the barrel the lands stand out. What do I need to measure on this casting?

What dimension do I need for the core to be for my finished paper patched bullet to be .285? 

 

Tom Stone

Lifetime NRA since 1956, NRA Benefactor, USN Member, CBA Member

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Ed Harris posted this 19 February 2019

Make the bullet fit the diameter of the unrifled portion of the chamber leade or throat, prior to the origin of rifling. 

Many posters on the other forum confuse the issue because they use "bore" to represent "groove" diameter as if they didn't know there was a difference.

BORE is the diameter of a drilled and reamed smooth bore barrel, such as a shotgun tube.

When bore is used to describe a rifled barrel it represents the diameter measured across the top of the rifling lands.

GROOVE diameter is the circumference subtended by the depth of grooves after the rifling has been cut. 

 

73 de KE4SKY In Home Mix We Trust From the Home of Ed's Red in "Almost Heaven" West Virginia

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RicinYakima posted this 19 February 2019

"In reading about paper patching, writers on the "other" CB forum have advised " do a pound cast  and make your boolit .001" to .0015" over BORE diameter" (their emphasis, not mine). " 

Ed is correct for the correct place to measure and 0.001" is plenty, with wheel weights or equal with soft alloy. Never trust people who can't spell.

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Ken Campbell Iowa posted this 19 February 2019

the BEST way to measure your slug's * bore * impression  is with an optical instrument ... if you know a machinist who likes to shoot, see if he can use an optical comparator  .... or if you know someone who does macro-photography, i would think a scheme could be created to overlay a image of your slug onto a known grid ....  images can be magnified in photoshop to a ridiculous extent ... actually count the pixels...  for a given camera sensor, you could take a picture of a known shim stock and determine the pixel size ....  whew ... ( g ) ...

but a direct approach is to stick different diameter pins down the barrel until they give some resistance ...  remember the muzzle diameter may be different from the bore just ahead of the chamber.  gunsmiths will have a couple hundred pins of about a thousandths different in diameter to do this with. 

and then there is what most of us casters really do ... just chamber a bullet out real long and see how much of the bore-riding front of the bullet engraves ...  too much and you can't chamber them and they dump the powder when pulling out an unfired one, and so we usually settle for seeing just shiny marks on the bullet nose.  when you just see shiny smooth marks, your bullet nose is within 1/2 thousandths of your bore diameter ... at the chamber end ...

************

for match shooting, some do use bullet noses that do stick and can't be unchambered cleanly .

hope this helps, kinda a hassle, but so far this bullet fit thing is RULE ONE  ...  and the next rule starts at about 11 or 14 in importance ...

ken

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harleyrock posted this 19 February 2019

Ken Campbell said:

 if you know someone who does macro-photography, i would think a scheme could be created to overlay a image of your slug onto a known grid ....  images can be magnified in photoshop to a ridiculous extent ... actually count the pixels...  for a given camera sensor, you could take a picture of a known shim stock and determine the pixel size ....  whew ... ( g ) ...

I have a digital microscope that I use to determine BHN.  I will try to measure the pound cast with that.

but a direct approach is to stick different diameter pins down the barrel until they give some resistance ...  remember the muzzle diameter may be different from the bore just ahead of the chamber.  gunsmiths will have a couple hundred pins of about a thousandths different in diameter to do this with.   

I have such a set of pins.

We have now pretty much settled how to measure the casting, but which of the diameters do I use as my core:  The "bore" as described in my quote from the other forum?  

Or most likely what Ed said 

"Make the bullet fit the diameter of the unrifled portion of the chamber leade or throat, prior to the origin of rifling." I will measure that with my digital microscope.

"Houston we have a solution"

Tom Stone

Lifetime NRA since 1956, NRA Benefactor, USN Member, CBA Member

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harleyrock posted this 19 February 2019

I have taken a photo with my digital microscope but it is too large to drag and drop onto this post.  It is a .jpg taken at 1600X1200 pixels.  I have compressed it to a .zip but I still can't drag and drop onto this page.  I am sure many of you know how to add an image to a post.  If you will coach me I would love to share the picture with you.

I have measured the unrifled portion of the pound cast using this micrograph and come up with .2776".

 

Tom Stone

Lifetime NRA since 1956, NRA Benefactor, USN Member, CBA Member

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Brodie posted this 20 February 2019

Tom,

I can't help you post your picture.  I wish that I could, but if you are going to try paper patching I would suggest that you make you core 1 to 2 thousands larger than the BORE (space between the lands) or .oo1 to .002 " larger than the largest pin that can fit down the barrel of your rifle.  This is necessary to make sure that the lands can cut completely through the paper.  I like to wet wrap, usually with tracing paper about .002 " to .0025 " thick.  this gives you a .008 to .010 ' increase in bullet diameter.  When fired there is generally a cloud or puff of white smoke which is the paper exploding away from the bullet.  Softer cores are better and in the cartridge you are interested in I would suggest pure lead or lead with 1% tin added.  Sizing of the wrapped bullet is best done in a push through die like the ones Lee makes or NOE.  I think that the NOE sizing set up is the better deal.  Those sizers are easily enlarged with a mandrel and some 300 to 600 grit wet dry paper.

I got interested in paper patching because I could shoot a soft lead bullet or soft alloy at moderate jacketed speeds for hunting, and from what I have seen they work quite well.

B.E.Brickey

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Ken Campbell Iowa posted this 20 February 2019

the best way to post pics here is to have it in your computer, and not too big .. but 1 meg is ok but not better than a half meg ..   go to the message box and after your text, .... click on the little icon that looks like a mountain scene ... another little box drops down and you hit * browse * ... it will open your computer ... then just click on the pic file in your computer  .. that will load it into the post here .  now you can size it down a little to be neat by putting your cursor on a little square on any corner and drag it towards the center  .. might take a few tries, it is hard to catch the little square every time.  your pic has to be a jpeg/jpg file .  you can't copy y paste .

hope this helps.  there is a practice thread here somewhere, but you can edit it later even in the current thread if you screw up .

ken

 

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Scearcy posted this 20 February 2019

Brodie

If you can email that picture feel free to send it to me and I will post it for you.

PM me if you would like to try this.

Jim

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harleyrock posted this 20 February 2019

Thanks to Ken Campbell here is my digital microscope macrograph:  The scale is 0,10 inch.

7X57 pound cast with 0.10 inch scale

Lifetime NRA since 1956, NRA Benefactor, USN Member, CBA Member

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harleyrock posted this 20 February 2019

I have measured the unrifled section with my micrometer.  It measured .2776"

So I need a core of that dimension and enough paper thickness in 2 wraps to yield a diameter of the groove diameter of .2846 (as measured from the rifled section of this cast) plus .001 to give a paper patched bullet with a diameter of .2856".  Is that correct?

 

Tom Stone

Lifetime NRA since 1956, NRA Benefactor, USN Member, CBA Member

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Chargar posted this 20 February 2019

I agree that measuring the "bore" offers little useful information for the cast bullet shooter, but if you want to do it, a machinest expanadble hole guage is just the tool. These are made by most all quality tool maker. I have a full set of Starrett hole gages, tht are used mostly to measure revolver cylinder throats. 

 

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Larry Gibson posted this 20 February 2019

harleyrock

Possible misunderstanding on the question here; possibly me but I'll answer what I think is the question. 

I believe they were correct on that other site.  You do want the bullet to be .001 - .0015 larger than the bore diameter (not confusing bore diameter with groove diameter).  The PP should then bring the diameter of the PP'd bullet up to the diameter of the throat or groove diameter which ever is larger.

The reason the bullet should be slightly larger than bore diameter so the rifling evenly cuts/shreds the PPing which is filling the grooves so the PPing is shed evenly on muzzle exit.

LMG

Concealment is not cover.........

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Ed Harris posted this 21 February 2019

I somehow mis-read the OP and didn't realize he was paper patching.  Senior moment...

73 de KE4SKY In Home Mix We Trust From the Home of Ed's Red in "Almost Heaven" West Virginia

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harleyrock posted this 21 February 2019

What part of the pound casting in the picture above is the "throat"?  I have been confused about the many names for that part of a gun from the case mouth to some point down the barrel.  I know what the muzzle is

What is "ball seat", "leade" "freebore"  "throat" etc. and whatever other names I have failed to mention?

Tom Stone

Lifetime NRA since 1956, NRA Benefactor, USN Member, CBA Member

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Larry Gibson posted this 21 February 2019

Appears to he a very generous chamber with a very large neck portion of the chamber, possibly a milsurp 7x57? 

All of those terms are inclusive in the term "throat" and are, often, meant to be the same thing.  However, they are not really the same thing. 

The throat in the picture begins ahead of the case where the pound cast rounds off and then is straight to the beginning of the rifling. 

The mouth of the throat is that part that rounds off ahead of the case and looks a bit like a shoulder.  The mouth is considerably more pronounced in your chamber probably because it is a milsurp chamber?

The freebore or ball seat is that straight (may have a slight taper to it if it is long) portion from the mouth to the beginning of the rifling.

The leade is from where the rifling begins tapering forward to bore diameter or where the taper stops.

That is basically the distinction between those terms.  However, you will see them used incorrectly and interchangeably in respected publications and by respected writers.  Unfortunately there doesn't seem to be a "Webster's Dictionary" of ballistic terminology.  Even in dictionarys we find the terms used interchangeably and even incorrectly.

For a proper "fit" of your PP'd bullet to the throat it should be of diameter sufficient to slide tightly into the freebore part of the throat to the leade without damaging the PP.  In recent times, on an other forum, we find "fitment" to be a term used to describe the correct fit of a bullet to the throat.  But that term is also incorrect as it is define properly as "1. fittings  2. equipment  3. furniture.  Many have their own idea of what the terms mean. 

The problem with different ideas of what the terms are and not using the correct (same if you will) terminology is confusing when, especially when giving advise.  We, the reloading fraternity, demad precise information regarding loads, powders, charges, bullet weights, diameters, etc. yet we can't seem to use the correct terminology on things such as this.  Strange eh?  

LMG 

 

Concealment is not cover.........

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harleyrock posted this 21 February 2019

Yes, strange indeed, and confusing.

The chamber is not a milsurp.  It is a post '64 Winchester model 70 Featherweight.  I am sure the deficiencies  in that pound cast are all mine.  I really haven't gotten good at that yet.  Ed Harris's method is much better than the one I used for this one.  If I can't get a good result from all this I will likely re-do the cast a la Ed Harris.

Tom Stone

Lifetime NRA since 1956, NRA Benefactor, USN Member, CBA Member

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