Copper springback in Match jacketed bullets

[DShortt]

An interesting idea was recently raised in a thread surrounding barrel slugging involving the idea that a copper jacket could 'spring back' thereby making the bore diameter of a centerfire rifle barrel shooting jacketed bullets not as critical as a rimfire barrel or a barrel shooting a pure lead slug.
I've never heard about this before and haven't turned up anything helpful in my online search. Is there someone here who can expound on this or perhaps direct me to a discussion/article on the subject?

Thanks for looking.

 

[fkimble@charter.net]

The jacket, I reckon could spring back, but the lead core doesn't spring back, so you would then have a "loose" lead core, if the jacket springs back. That doesn't sound good for accuracy. Of course bullets use gilding metal, not copper for jackets.

Frank

 

[Steve Blair]

The jacket, I reckon could spring back, but the lead core doesn't spring back, so you would then have a "loose" lead core, if the jacket springs back. That doesn't sound good for accuracy. Of course bullets use gilding metal, not copper for jackets.

Frank

Gilding metal is 88-95% copper. It behaves similarly. More would depend upon the material condition of the jacket, even if this unlikely effect were true.

 

[RegionRat]

The focus of barrel slugging is to substitute a piece of soft lead for an expensive custom air gage which is what is normally used to investigate barrel quality in a well equipped facility. Long ago, folks used soft lead to evaluate bore diameters and so it continues as an art.

The "feel" of the loose or tight spots is what comes out of slugging a bore and it isn't reliable or useful for the fourth digit (inches). Folks will often go ahead and make measurements, but they know those values are risky when discussing ten-thousandths of an inch but still more than good enough to spot issues.

Gilding alloy metal jackets (C21000 but not always) on cup-core bullets are very malleable too, but not like lead. It is roughly 5% zinc.

The advantage of lead is it can be obturated in place using a reverse shaft of wood or metal in the case where it was driven past a middle "tight" spot such that it would then be too undersized for the balance of the test.

With soft lead, you obturate the slug in place so it expands in diameter and you can then continue further down the barrel without a loose slug.

The book by Calfee on rimfire accuracy has some words on the topic if you like to read that sort of thing.
https://www.amazon.com/Art-Rimfire-Accuracy-Bill-Calfee/dp/145679776X

The pressures of firing cycles in firearms and even most air rifles, tends to obturate the bullet. Depending on the context, some amount of blow-by leakage is present. If a barrel diameter is oscillating from tight to loose, or loose toward the crown, nothing good comes from it. So, smiths have always been interested in knowing the quality of a barrel bore before pouring time into it.

 

[DShortt]

The focus of barrel slugging is to substitute a piece of soft lead for an expensive custom air gage which is what is normally used to investigate barrel quality in a well equipped facility. Long ago, folks used soft lead to evaluate bore diameters and so it continues as an art.

The "feel" of the loose or tight spots is what comes out of slugging a bore and it isn't reliable or useful for the fourth digit (inches). Folks will often go ahead and make measurements, but they know those values are risky when discussing ten-thousandths of an inch but still more than good enough to spot issues.

Gilding alloy metal jackets (C21000 but not always) on cup-core bullets are very malleable too, but not like lead. It is roughly 5% zinc.

The advantage of lead is it can be obturated in place using a reverse shaft of wood or metal in the case where it was driven past a middle "tight" spot such that it would then be too undersized for the balance of the test.

With soft lead, you obturate the slug in place so it expands in diameter and you can then continue further down the barrel without a loose slug.

The book by Calfee on rimfire accuracy has some words on the topic if you like to read that sort of thing.
https://www.amazon.com/Art-Rimfire-Accuracy-Bill-Calfee/dp/145679776X

The pressures of firing cycles in firearms and even most air rifles, tends to obturate the bullet. Depending on the context, some amount of blow-by leakage is present. If a barrel diameter is oscillating from tight to loose, or loose toward the crown, nothing good comes from it. So, smiths have always been interested in knowing the quality of a barrel bore before pouring time into it.

That's what I've read in several places but recently I've come into information from more than one source that the idea of a tapered bore, particularly a bore that tapers toward the muzzle is only relevant in rimfire barrels and not applicable to centerfire barrels with jacketed bullets. I can neither prove nor disprove such claims. Hence the reason behind this thread.

 

[BoydAllen]

The focus of barrel slugging is to substitute a piece of soft lead for an expensive custom air gage which is what is normally used to investigate barrel quality in a well equipped facility. Long ago, folks used soft lead to evaluate bore diameters and so it continues as an art.

The "feel" of the loose or tight spots is what comes out of slugging a bore and it isn't reliable or useful for the fourth digit (inches). Folks will often go ahead and make measurements, but they know those values are risky when discussing ten-thousandths of an inch but still more than good enough to spot issues.

Gilding alloy metal jackets (C21000 but not always) on cup-core bullets are very malleable too, but not like lead. It is roughly 5% zinc.

The advantage of lead is it can be obturated in place using a reverse shaft of wood or metal in the case where it was driven past a middle "tight" spot such that it would then be too undersized for the balance of the test.

With soft lead, you obturate the slug in place so it expands in diameter and you can then continue further down the barrel without a loose slug.

The book by Calfee on rimfire accuracy has some words on the topic if you like to read that sort of thing.
https://www.amazon.com/Art-Rimfire-Accuracy-Bill-Calfee/dp/145679776X

The pressures of firing cycles in firearms and even most air rifles, tends to obturate the bullet. Depending on the context, some amount of blow-by leakage is present. If a barrel diameter is oscillating from tight to loose, or loose toward the crown, nothing good comes from it. So, smiths have always been interested in knowing the quality of a barrel bore before pouring time into it.

People still slug bores and cast laps for measuring groove diameter and finding tight and loose spots. Some time back I spoke with Frank Green (owner of Bartlein) about this and he agreed that you can feel things with a cast lap (which would also be true for a soft lead slug) that you cannot detect with an air gauge. There is nothing out of date with the practice. A friend has found tight and loose spots in expensive lapped barrels, so he now makes a practice of casting a lap to feel for these and measuring groove diameters before starting work on new barrels. He only works with top grade barrels. He also finds this quite useful for diagnosing accuracy problems of rifles that come to him with those problems.

 

[DShortt]

Huh.

 

[DShortt]

Perhaps it was tongue in cheek?