Overbore

[NUMBERFARMER]

I have seen the term tossed around in this forum and others that I visited before I landed here.  Can someone please give me the definition for overbore?  It would seem that powder charge divided by the area of the bore would enter into the equation, but barrel length would have a tremendous amount to do with utilizing case capacity.  If a caliber is overbore, what standard barrel length is that based on, and what is the ratio of caliber to case capacity? How can a cartridge be overbore if it can utilize all the powder in the case in say a 26" barrel?Since 16" is the minimum legal rifle barrel length, wouldn't any cartridge that needs a barrel longer than this be overbore?  Why is a 25-06 considered overbore just because it needs a 26" barrel to fully utilize case capacity? All right, I've confessed my ignorance now lets have the anwers.

[LLANOJOHN (deceased)]

NUMBERFARMER,
 
OOPS!  MY APOLOGIES HERE........I GOT YOU POST MIXED UP WITH ANOTHERS CONCERING THE 30-06 AI...BUT THE INFO PRETTY WELL APPLIES TO BOTH POSTS!
 
OJIH
 
Here is a quote from the chapter on "Bore Capacity", Vol. 1, Handbook for Shooters & Reloaders by P. O. ACKLEY.
 
"The ratio between bore and cartridge case capacity is a subject about which more is heard these days than ever before. The term "over bore capacity" is applied to many cartridges which is to say that an extremely large cartridge holds more powder than the bore can handle efficiently. Powley ties up bore capacity with expansion ratio. Expansion ratio is the ratio between the total volume of the bore and the volume of the case. It is the number of time that the gas will expand by the time the bullet reaches the muzzle. High expansion ratios mean good barrel life. Low ratios mean short barrel life. Low ratios may result in the highest velocities but in many instances this slightly higher velocity is more than offset by greatly reduced barrel life, critical loading and general inflexibility."
 
As most of the long-time viewers on this forum know I am a great admirer of P. O. (Parker Otto) Ackley! I am but one of the many admirers of this gentleman gunsmith and barrel maker. Mr Powley, as noted in the above quoted paragraph was an eminent ballistician of the same period as Ackley.
 
..............................................................................................
quote same source as above
 
P O Ackley was graduated from Syracuse University, Magna cum laude, in 1927 and entered the full-time gunsmith business in Roseburg, Oregon in 1936. This enterprise flourished, but in 1942 war work beckoned and he became attached to the Ordinance Department and was with the small arms section in Ogdon, Utah until 1944. He re-established his gun shop in Trinidad, Colorado in 1945 and had one of the largest custom gunshops in the nation. He was instructor in theory of gunmaking and metallurgy at Trinidad State Junior College from 1946 until 1951, and carried on extensive experimental work at the college and in his own gunshop which had been incorporated in 1947.
 
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As you will note P O Ackley started his gunshop during the "Great Depression" and my personal belief is that this greatly influenced his designs and thoughts on balanced or "efficient" cartridges. In other words, "efficient" equals the "most BANG" for the amount of "BUCKS" invested.
 
One more quote from his book and I will end this post and wait for a continuing discourse!
 
"He has built a reputation for 'always telling the truth' in his writings."
 
Ol' John

[LLANOJOHN (deceased)]

OK, now we have a reference point with which to begin this discussion.
 
A cartridge such as the 30-06 AI which you originally started with, would be in my estimation about just right for efficiency and not "over bore" with a barrel of 24".  This probably would a good combination with H-4350 or AA4350 coupled with the 165-168 grain bullets you stated.  Good barrel life, most "BANG" for the "$$$$", long case life and "not too twitchy" during load development to determine a very accurate load.  You might be able to gain more FPS with a 26" barrel and using a powder such as H-4831-SC.  Will the 26" barrel be suitable for what you want to do or will you have to limit the barrel length to 24".  If this is to be a dedicated target rifle then 26" or possibly 28" would be considered.  If it is to be a hunting/target rifle, then depending on hunting terrain and distance traveled, 26" might be the maximum length.
 
Just some random thoughts for you to consider.
 
Ol' John

[NUMBERFARMER]

Ok so you have basically verified what I thought was the basis of the equation. But what is the line, what is the ratio that determines overbore. How many rounds does a barrel need to fire without wearing out before the cartridge itself is no longer considered overbore. Surely there is some mathmatical formula that defines this topic since so many are so forceful about the subject.
 
What definition exists to draw the line that puts some cartridges on the overbore side and others on the efficient or maybe even underbore side.
 
From an intuitive point of view it would appear that any cartridge that can fire a heavy for caliber bullet at much over 2500 - 2700 fps (rough guess) would be considered overbore.
 
For example, a 22 Hornet must fall in the underbore category, while a 223 would seem to be slightly overbore. The 308 and 30-06 would appear to by close to the line with one slightly under and the other slightly over.
 
I am looking for the equation that says a cartridge with bore diameter y and case volume x is over bore if x to the N is bigger than y times z. Does this type of equation really exist or is overbore just an opinion without any baseline to develop a consisten assesment of cartridges.

[drinksgin (deceased)]

NF;

It is not very scientific, but i have been told, if you shoot it after dark and it looks like a volcano, the barrel is too short, the powder is too slow or it is overbore.
:D
Don

[Gerard]

It is doubtful whether overbore can be defined as a line to be crossed. As an example, a given cartridge could be overbore or not, by virtue of using  different powders. I have accepted the term as a loose description of a combination of circumstances that do not work very well. Also, what was considered overbore 5 or ten years ago, is now perfectly workable and has resulted in some cartridges that are a very real pleasure to use.

[LLANOJOHN (deceased)]

NUMBERFARMER,
 
I have spent some time researching to see if I could locate some valid info or formula to determine "IDEAL" case capacities for various calibers. No luck .... 'xcpt for the .22 caliber and the caliber I have the most experience....the .26 or 6.5 caliber.
 
For the .22 caliber it is generally agreed that 35 grain capacity is about right.
 
For the .26 caliber it is likewise agreed that something like a 55-57 grain capacity.
 
The only load software, to my knowledge, that uses the Homer Powley system is "Load From A Disc". I have it and found it very helpfull in load development and powder selection. Since the software has a cartridge "designer" program, it was most helpful when Gitano and I were developing the .338x57 MAI wildcat cartridge.
 
Way back in the '60s when the Powley system came available you operated a modified style of a sliderule which also required pencil and paper to be available. This one time when I am most happy that times have changed for the better.
 
I would suggest you invest in the 2 books.."HANDBOOK FOR SHOOTERS & RELOADERS, VOLUMES 1 & 2" by P. O. Ackley. the copyright on my copies is 1962, the 21st printing of 1992. The load data is pretty much outdated but you can use it to make comparisons.
 
If you are interested in cartridge design and so on...."Load From A Disc" is relatively inexpensive somewhere in the $60 range if memory serves me correctly.
 
I hope this discussion helps and we can certainly continue the discussion. I am still researching to see if I can find helpful information.
 
Ol' John

[drinksgin (deceased)]

John;
An aside, what is the difference between the .338x57 you and Paul developed and the .338x57 that O'Connor proposed but as far as I know, never had one built?
Don

[LLANOJOHN (deceased)]

John;
An aside, what is the difference between the .338x57 you and Paul developed and the .338x57 that O'Connor proposed but as far as I know, never had one built?
Don

Hey neighbor!
 
Paul and I took the cartridge one step farther and did the Ackley Improved, hence the 338x57 MAI (Mauser Ackley Improved). Since I know next to -0- about 338 calibers Paul was of the opinion the bullets necessary for Elk, Caribou and others would be the Nosler 200gr Ballistic Silvertip and the Nosler 210gr Partition. The whole idea was a good cartridge of proper length to fit the '98 Mauser action. My particular rifle is 95% completed and Pauls is still in process. We are looking at velocities of approximately 2600fps with 24" barrels. We both use the Czech VZ-24 actions, mine has a ER Shaw SS and Paul's barrel is a 24" C/M by Lothar Walther.
 
Did I answer your question or is there other info you might require?
 
Ol' John

[drinksgin (deceased)]

John;
Thanks, it will be very interesting to see what sort of results you get actually shooting it.
Don

[NUMBERFARMER]

I am still pondering all that has been said.  I really thought that overbore must be something easily defined since many people have used it to criticize my choice of firearms (not necessarily on this forum).  It would appear that the definition of overbore is about as elusive as MAGNUM.