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Overbore cartridges

Mark204

So some time ago I read the article on overbore cartridges, yes it's interesting, something we could probably have endless pages of debate on. Well I saw it again and like last time I played with the math with no results.

My question is....what is the mathematical formula to figure this out? I'm going to use the .220 Swift as an example........It case has 48 GC, .224 BD and the case volume to bore area ratio is 1218.0 I understand the 48GC, I understand the .224 BD..............But how does one get to the 1218? Can someone lay the math out for me using the cartridge listed above?

I don't need this for any specific reason other than the fact that I want to understand the "how" of it.

Thanks in advance,

Mark
 
Until powder capacity reaches a point where any increase in capacity results in ZERO velocity increase, it’s not overbore.

Any other discussion on tbe subject is nothing more than an arbitrary definition efficiency.

Now this old pissing match re-kindles once again...
 
Last edited:
Tommie said:
Until powder capacity reaches a point where any increase in capacity results in ZERO velocity increse, it’s not overbore.

Any other discussion on tbe subject is nothing more than an arbitrary definition efficiency.

Now this old pissing match re-kindles once again...
Click to expand...


Esoteric Junkie said:
Tommie is right on point with his above comments. There is no such thing as an overbore cartridge! Any discussion regarding "overbore" cartridges is purely academic.

I like cartridges that burn egregious amounts of powder:D:D:D. Barrel makers have to eat to!

EJ
Click to expand...

I think it’s fairly easy to find an overbore cartridge.

Compare the 300 Win Mag to the 30-378 Weatheyby Mag both shooting 180 grain bullets from Nosler data... the 300 tops out at around 3120 with 80 grains H1000. The 30-378 gets a whopping 3180 burning 100 grains of the same powder. The volume of gas generated is 20% higher but you only get 2% more velocity. Efficiency is in the trash can.

If I use a car engine as an analogy it might be clearer...
If you had an efficient engine the last thing you would do is add combustion chamber volume without an increase in stroke. The reason is that you’d have to add fuel just to get back to what you had before then you’d have to add fuel to get a small increase in power.
 
riflewoman said:
I think it’s fairly easy to find an overbore cartridge.

Compare the 300 Win Mag to the 30-378 Weatheyby Mag both shooting 180 grain bullets from Nosler data... the 300 tops out at around 3120 with 80 grains H1000. The 30-378 gets a whopping 3180 burning 100 grains of the same powder. The volume of gas generated is 20% higher but you only get 2% more velocity. Efficiency is in the trash can.

If I use a car engine as an analogy it might be clearer...
If you had an efficient engine the last thing you would do is add combustion chamber volume without an increase in stroke. The reason is that you’d have to add fuel just to get back to what you had before then you’d have to add fuel to get a small increase in power.
Click to expand...

Compared to a 30-06, the 300 Win Mag is overbore?
And the 30-06 is overbore compared to the 30-30 Win?
And then there's the 30 Carbine.

Case capacity is always subject to the law of diminishing returns. But there is a point where more capacity results in zero velocity increase and THAT is the point where the overbore condition has been reached. And also recall that previously "overbore" cartridges have been given a reason for being as powder technology advanced. As I stated, anything less than true "overbore" is merely a discussion of relative efficiency.

I'm not sure about the engine analogy. Increasing the size of the combustion chamber basically just reduces compression ratio.
 
Tommie said:
Compared to a 30-06, the 300 Win Mag is overbore?
And the 30-06 is overbore compared to the 30-30 Win?
And then there's the 30 Carbine.

Case capacity is always subject to the law of diminishing returns. But there is a point where more capacity results in zero velocity increase and THAT is the point where the overbore condition has been reached. And also recall that previously "overbore" cartridges have been given a reason for being as powder technology advanced. As I stated, anything less than true "overbore" is merely a discussion of relative efficiency.

I'm not sure about the engine analogy. Increasing the size of the combustion chamber basically just reduces compression ratio.
Click to expand...
And compression ratio is the inverse of expansion ratio. So a 7:1 compression ratio is more efficient than a 4:1 ratio. An expansion ratio of 7:1 like an 06 is more efficient than a 4:1 of the 30-378. Now if you add barrel length like say 40 inches the 30-378 becomes much more efficient.
 
riflewoman said:
I think it’s fairly easy to find an overbore cartridge.

Compare the 300 Win Mag to the 30-378 Weatheyby Mag both shooting 180 grain bullets from Nosler data... the 300 tops out at around 3120 with 80 grains H1000. The 30-378 gets a whopping 3180 burning 100 grains of the same powder. The volume of gas generated is 20% higher but you only get 2% more velocity. Efficiency is in the trash can.

If I use a car engine as an analogy it might be clearer...
If you had an efficient engine the last thing you would do is add combustion chamber volume without an increase in stroke. The reason is that you’d have to add fuel just to get back to what you had before then you’d have to add fuel to get a small increase in power.
Click to expand...

Sounds like we are comparing a 300 Win that is loaded hot to a 30-378 that is barely idling along.
If I’m racing cars.... I don’t care how efficient it is or how much fuel I burn - I want to get from point A to point B as fast as possible.
 
Gary in MD said:
Sounds like we are comparing a 300 Win that is loaded hot to a 30-378 that is barely idling along.
If I’m racing cars.... I don’t care how efficient it is or how much fuel I burn - I want to get from point A to point B as fast as possible.
Click to expand...
You might want to consult Kurt Busch on racing and fuel efficiency.
 
JimT said:
You might want to consult Kurt Busch on racing and fuel efficiency.
Click to expand...
For anyone interested in a reasoned, educated and interesting article on this subject I refer you to P.O. Ackleys Handbook For Shooters & Reloaders, Vol. 1. Apparently Riflewoman has read it and most others not.
 
JimT said:
For anyone interested in a reasoned, educated and interesting article on this subject I refer you to P.O. Ackleys Handbook For Shooters & Reloaders, Vol. 1. Apparently Riflewoman has read it and most others not.
Click to expand...

I have both volumes..... well worn from reading over the years.
SOME of his stuff is still relevant today.
Gary
 
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