Comments on: Defining “Overbore” Cartridges via Comparative Index

By: david joe

david joe — Thu, 06 Mar 2014 16:15:08 +0000

… Following my previous post, not only does bore volume seem the appropriate factor since that’s how much space the case’s “volume” of powder can expand in, (not internal barrel wall area) but further still, the 50 BMG, tagged here as the most overbore cartridge, is not particularly a high pressure round in this company, nor does it burn out barrels fast, nor have we seen the military utilize a round that is either inefficient (wastes powder) or is relatively hard on barrels, rather the opposite. Common sense tells me that a machine gun bore firing 7mm Mag shells will last nowhere near as long as the Browning BMG.

By: david joe

david joe — Thu, 06 Mar 2014 15:51:15 +0000

If the volume of the case is being considered, then why not the “volume” of the bore, instead of the “area”? Sure, greater cross-section area between two bores also implies greater volume, but not proportionately so. If you simply look at a 50 BMG case and also the rifle’s bore, it defies observational intuition that it is the “most overbore” cartridge on this list.

By: Tommie

Tommie — Thu, 06 Mar 2014 01:18:38 +0000

Overbore: when increased capacity yields no additional performance.

All else is a discussion regarding relative efficiency.

By: Tommie

Tommie — Wed, 05 Mar 2014 02:10:44 +0000

Overbore: when the increased capacity results in no performance gain regardless of powder type, bullet weight, etc etc.

Everything else is an argument over subjective definitions of efficiency. Period.

By: Jeff

Jeff — Tue, 04 Mar 2014 01:27:21 +0000

The whole ‘overbore’ discussion is a bit hypothetical for the vast majority of shooters. The .264 Mag got a lot of bad press when it was around en mass for being a ‘barrel burner’. Probably is if you shoot it a lot but the average hunter chasing speed goats shoots very few rounds.

By: Josh

Josh — Mon, 03 Mar 2014 01:07:10 +0000

We should all thank those behind this website for occasionally publishing provoking articles. Unfortunately, this one seems to be mostly opinion, without much science to back it up.

The term, “Ballistic Efficiency”, has been available for years. It is the percentage of the chemical energy stored in the powder charge which is converted to kinetic energy of the bullet. The rest of the stored energy goes to noise, heat, recoil, etc. The lower the efficiency, the more wasted energy, and the more “Overbore” the cartridge

Quickload software (reviewed on this site) returns Ballistic Efficiency as a calculated value.

As examples, using maximum loads from the Hodgdon online loading data:
222 Remington, 50 gr. bullet, 20.5 grs H4198, MV=3160 fps
30-06 Springfield, 180 gr bullet 57.5 grs H4350, MV=2798 fps

From Quickload 3.2, using the same loads, and a bullet seating depth which yields the same MV as Hodgdon:
For the 222, Ballistic Efficiency = 29%; Overbore Number from this article = 685
For the 30-06, Ballistic Efficiency = 29%; Overbore Number from this article = 915
Hodgdon has no 50 BMG data, but typical Quickload efficiencies are 23-26%; Overbore Number = 1480

By: eeb

eeb — Sun, 02 Mar 2014 23:37:45 +0000

Useful as a comparison guide, though other factors need to be taken into consideration.

By: Robert

Robert — Sun, 27 Nov 2011 12:52:39 +0000

D Pepper,
That back issue of Handloader is sold out and I cannot find it anywhere. Do you have that article you mentioned?

By: Al Nyhus

Al Nyhus — Wed, 13 Apr 2011 22:56:44 +0000

An interesting adjunct to this chart would be to publish the exspansion ratios for each case. You’d need to use a barrel length appropriate for each cartridge, of course.

I’ve run a few examples and the relationship between the ‘Index’ published here….and actual exspansion ratios….are interesting.

By: Roarke

Roarke — Wed, 13 Apr 2011 16:54:13 +0000

The chart is interesting and ideally you’d be able to graph several different cartridge specifications all on top of each other to see where the lines intersect – assuming you had some accuracy constant on the x axis and all of these other factors (overbore, case capacity, shoulder angle, etc) on the y axis. But therein lies the issue – there is no accuracy specification for any cartridge, let alone several dozen so you’re unable to analytically relate any of these others factors to some standard.
The real issue is finite shooting budgets!!