Overbore cartridge formula
- Thread starter 6mmdasher
- Start date
Greg Culpepper
New member
6mm,
Overbore is a concept and not a law. Opinions vary as to where overbore starts. A 308 Win guy might think a 30-06 is overbore. Rimfire guys think all centerfires, even a Hornet are overbore. Case volume to bullet base area and case volume to bore volume are two ratios that can illustrate the concept. Then you can apply your own threshold for when overbore begins as you see fit.
Greg
Overbore is a concept and not a law. Opinions vary as to where overbore starts. A 308 Win guy might think a 30-06 is overbore. Rimfire guys think all centerfires, even a Hornet are overbore. Case volume to bullet base area and case volume to bore volume are two ratios that can illustrate the concept. Then you can apply your own threshold for when overbore begins as you see fit.
Greg
jackie schmidt
New member
Overbore
After looking at that chart, it is easy to see why 30BR barrels seem to last forever..........jackie
After looking at that chart, it is easy to see why 30BR barrels seem to last forever..........jackie
Larry Elliott
Old bald guy
A friend who is an inveterate wildcatter has a theory based entirely bullet weight to be used and case capacity. If the powder charge to be used is more than a certain percentage of the bullet weight, 75% or so as I remember, then the cartridge is overbore according to him.
Back in the 1930's when the .220 Swift came out it was considered to be overbore, as was the .25-06. With the powders available at the time they likely were. As newer slower and/or more progressive burning powders became available neither of those cartridges is considered to be overbore, at least grossly so as they were 60 or 70 years ago.
Obviously any cartridge larger than a certain capacity rapidly loses efficiency (fps/gr powder), and the larger they are the more rapid the falloff in efficiency is. In Cabrera's excellent (IMO) system it's obvious that the smaller .22's that are larger than the .222 are only slightly less efficient than the .222 while the Hornet and .221 are much more efficient.
A lot of this involves how long a shooter wants his barrel's accurate life to be, and what he wants his rifle to do. Shooting small varmints at very long range with a Hornet or .221 requires more luck, skill, and shots than it does with even a .223 or .22 BR.
Back in the 1930's when the .220 Swift came out it was considered to be overbore, as was the .25-06. With the powders available at the time they likely were. As newer slower and/or more progressive burning powders became available neither of those cartridges is considered to be overbore, at least grossly so as they were 60 or 70 years ago.
Obviously any cartridge larger than a certain capacity rapidly loses efficiency (fps/gr powder), and the larger they are the more rapid the falloff in efficiency is. In Cabrera's excellent (IMO) system it's obvious that the smaller .22's that are larger than the .222 are only slightly less efficient than the .222 while the Hornet and .221 are much more efficient.
A lot of this involves how long a shooter wants his barrel's accurate life to be, and what he wants his rifle to do. Shooting small varmints at very long range with a Hornet or .221 requires more luck, skill, and shots than it does with even a .223 or .22 BR.
Overbore cartridge quide
Dasher -
Howdy !
It's mostly about the " expasnion ratio ".....
Best place to go for answers, sre any articles authored by Homer Powley. Powley ( now deceased ) was a superb ballistician, with an Army Arsenal background ( I believe ).
Powley invented an internal ballistics calculator, which he had fabricated into a device known as the " Powley Computer ".
Powley's computer looked like a sort of thick cardboard slide rule.
It's was of great use to " wildcatters ".
Been a while since I manipulated one of those, but from memory:
- One would make data entries for things like proposed case capacity
- cartridge bullet calibre & wt
- barrel length
The computer would present an " expansion ratio " solution.
Expansion ratios that fell beyond a certain point, could be /would be considered " excessive ", or... characteristic of an " over-bore " design.
A rifles' expansion ratio can be altered by either:
- Shortening or lengthening of the barrel ( for a given calibre )
- Selecting/making the chamber of various volumetric capacities
In the case of using barrel length to adjust a gun/cartridge combo's expansion ratio is that seen by the penchant of long-range rifle shooters to use super-long barrels ( e.g. many chamberings used for 1,000yd shooting ).
For example: a shooter using a .300Win Mag going with a 30-32" barrel.
Ostensibly, the long barrels allow the larger powder charges time to more fully burn ( gasses to expand ). Conversely, short-range benchrest shooters operating 6PPC 's don't need much more than 20-22" barrel to push 65-69gr
6mm BR bullets into their velocity " Sweet spot ".
IF one is for some reason held to a specific barrel length for a given calibre, then the only way effectively alter the gun/cartridge combo's expansion ratio is to modify the chamber size. In some instances, rifle chambers can be / are re-cut to a accept a case with larger powder capacity.
When one is dead set on a certain calibre/bullet wt/ barrel length, it is then likely the gun can't be re-chambered for a smaller-capacity case; and the gun then has to be re-barrelled. In some instances, shooting competition class rules may themselves mandate pratical limits on rifle barrel length
( if for no other reason than for the shooter to be able to " make " a certain weight class cut-off point )>
Your questions run to...
- " How big a chamber is TOO big, for a given calibre/bullet wt " ?
- " How long a barrel ( for a given calibre/bullet wt ) is long enough " ?
Powley's formulas were amazingly predictive/accurate, for all center-fire rifle cartidge combos. His computer data could even be extrapolated to forecast performance of huge artillery, such as the German " Paris Gun "; with spot-on accuracy.
His computer utilized available IMR powders of the period, as base reference points; to allow shooters to determine where in the powder burn rate chart they should likely select powders from ( what " speed " powder ).
Alas, my specific memory about Powley's computer has become somewhat vague. But, it seems to me an expansion ratio value of " 1.75 " was still not classically " overbore ".
For example: As I recall, a rifle chambered in .22-250, with a 24" 1-14 .224" calibre barrel and shooting 55gr bullets... was able to muster a 1.75 value. Chamber the same notional rifle in .220 Swift; plug-in the numbers; and the computer rendered an " over-bore " determination.
When " fine-tuning " case capacities within a fixed calibre domain, one could take a borderline cartidge like A 6MM Remington; and come-up with bullet weights / barrel length combos that are alternately just right.. or over-bore...
dependent of computer operator's notional input values. " Plug-and-play ".
Sounds about right, to me.
Ken Water's " case capacity " tables were an excellent adjunct to the Powley Computer, allowing the wildcatters to more-easily know the starting capacity of their " basic " or " parent " brass choices.
I believe " Shooter Digest " once printed Ken's usefull capacity tables in the early 70's. Powleys Computer was on-sale during the same basic time frame.
Robert Hutton's chart of various bullet calibres/wts/ogives et al, was knowingly offered for sale, to also aid in the use of Powley's Computer when doing ballsitics calc.
Lastly.... barrel length cannot be expected to fix all over-bore conditions, as there are finite limits to how much barrel length any powder might be able to exploit; to provide maximum bullet speed for a given bullet calibre/wt.
I believe Powley's computer has since been updated for the internet age,
and can be had as a on-line resource; or a down-loadable variant.
I myself, am not able to tell you any such internet address(es ). Sorry !
Best of luck in your endeavors.
Regards,
357Mag
Dasher -
Howdy !
It's mostly about the " expasnion ratio ".....
Best place to go for answers, sre any articles authored by Homer Powley. Powley ( now deceased ) was a superb ballistician, with an Army Arsenal background ( I believe ).
Powley invented an internal ballistics calculator, which he had fabricated into a device known as the " Powley Computer ".
Powley's computer looked like a sort of thick cardboard slide rule.
It's was of great use to " wildcatters ".
Been a while since I manipulated one of those, but from memory:
- One would make data entries for things like proposed case capacity
- cartridge bullet calibre & wt
- barrel length
The computer would present an " expansion ratio " solution.
Expansion ratios that fell beyond a certain point, could be /would be considered " excessive ", or... characteristic of an " over-bore " design.
A rifles' expansion ratio can be altered by either:
- Shortening or lengthening of the barrel ( for a given calibre )
- Selecting/making the chamber of various volumetric capacities
In the case of using barrel length to adjust a gun/cartridge combo's expansion ratio is that seen by the penchant of long-range rifle shooters to use super-long barrels ( e.g. many chamberings used for 1,000yd shooting ).
For example: a shooter using a .300Win Mag going with a 30-32" barrel.
Ostensibly, the long barrels allow the larger powder charges time to more fully burn ( gasses to expand ). Conversely, short-range benchrest shooters operating 6PPC 's don't need much more than 20-22" barrel to push 65-69gr
6mm BR bullets into their velocity " Sweet spot ".
IF one is for some reason held to a specific barrel length for a given calibre, then the only way effectively alter the gun/cartridge combo's expansion ratio is to modify the chamber size. In some instances, rifle chambers can be / are re-cut to a accept a case with larger powder capacity.
When one is dead set on a certain calibre/bullet wt/ barrel length, it is then likely the gun can't be re-chambered for a smaller-capacity case; and the gun then has to be re-barrelled. In some instances, shooting competition class rules may themselves mandate pratical limits on rifle barrel length
( if for no other reason than for the shooter to be able to " make " a certain weight class cut-off point )>
Your questions run to...
- " How big a chamber is TOO big, for a given calibre/bullet wt " ?
- " How long a barrel ( for a given calibre/bullet wt ) is long enough " ?
Powley's formulas were amazingly predictive/accurate, for all center-fire rifle cartidge combos. His computer data could even be extrapolated to forecast performance of huge artillery, such as the German " Paris Gun "; with spot-on accuracy.
His computer utilized available IMR powders of the period, as base reference points; to allow shooters to determine where in the powder burn rate chart they should likely select powders from ( what " speed " powder ).
Alas, my specific memory about Powley's computer has become somewhat vague. But, it seems to me an expansion ratio value of " 1.75 " was still not classically " overbore ".
For example: As I recall, a rifle chambered in .22-250, with a 24" 1-14 .224" calibre barrel and shooting 55gr bullets... was able to muster a 1.75 value. Chamber the same notional rifle in .220 Swift; plug-in the numbers; and the computer rendered an " over-bore " determination.
When " fine-tuning " case capacities within a fixed calibre domain, one could take a borderline cartidge like A 6MM Remington; and come-up with bullet weights / barrel length combos that are alternately just right.. or over-bore...
dependent of computer operator's notional input values. " Plug-and-play ".
Sounds about right, to me.
Ken Water's " case capacity " tables were an excellent adjunct to the Powley Computer, allowing the wildcatters to more-easily know the starting capacity of their " basic " or " parent " brass choices.
I believe " Shooter Digest " once printed Ken's usefull capacity tables in the early 70's. Powleys Computer was on-sale during the same basic time frame.
Robert Hutton's chart of various bullet calibres/wts/ogives et al, was knowingly offered for sale, to also aid in the use of Powley's Computer when doing ballsitics calc.
Lastly.... barrel length cannot be expected to fix all over-bore conditions, as there are finite limits to how much barrel length any powder might be able to exploit; to provide maximum bullet speed for a given bullet calibre/wt.
I believe Powley's computer has since been updated for the internet age,
and can be had as a on-line resource; or a down-loadable variant.
I myself, am not able to tell you any such internet address(es ). Sorry !
Best of luck in your endeavors.
Regards,
357Mag
Water case volume
Water case volume is what I used, from different sources; reloading manuals, ammoguide.com, and some of my own measurements, i.e., water weight capacity up to the case mouth.
I can understand the rationale of using up to the shoulder-neck junction volume instead, but sorry that's not what I used.
And that was a good read about the Powley Computer that .357Mag provided.
With a quick search found it on the web:
http://kwk.us/powley.html
Good shooting,
George
Water case volume is what I used, from different sources; reloading manuals, ammoguide.com, and some of my own measurements, i.e., water weight capacity up to the case mouth.
I can understand the rationale of using up to the shoulder-neck junction volume instead, but sorry that's not what I used.
And that was a good read about the Powley Computer that .357Mag provided.
With a quick search found it on the web:
http://kwk.us/powley.html
Good shooting,
George