What is "overbore"

Boy, that's an arbitrary term if ever there was! The first mention of overbore I came across was in P.O. Ackley's handbooks, and it basically meant the powder capacity of the case was too much for the bore diameter of the barrel. Further increases in powder capacity, while providing increases in velocity, did so with a diminishing return the further overbore one went. Case in point: the 6.5x55 Swedish Mauser peaks around 50 grains of IMR4350 to launch a 120gr bullet at 2880fps. Going up, my 6.5-06 uses 55 grains of IMR4350 to move that same bullet at 3200fps. That's a 5 grain powder increase for a 320fps velocity increase. The .264 Winchester Magnum has been loaded with 62 grains of IMR4350 to push the same 120gr bullet to 3350fps. The powder increase is 7 grains, with a velocity increase of 150 fps. And they go higher yet, to include the 6.5-300 Weatherby Magnum, and the 6.5 STW. But notice that even as you push the bullets faster, the powder increase isn't linear with the velocity increase, you get to the point where even a goodly amount of powder increase effects only a small rise in velocity. Ackley referred to the rounds that went past optimum efficiency for their powder capacity as "overbore". Now, take my examples above with a bit of salt, because there are other factors, to include the pressures at which the rounds operate, barrel length, case design (PPC), etc. Nobody would ever call the 6.5x55 Swedish Mauser a high pressure round, while the 6.5 STW definitely falls into that category. Here's another comparison: The .30-06 is considered to be "efficient", while the .30-378 Weatherby Magnum is "overbore". Can you see the distinction?

(Gawd, I'm long-winded, sorry about that!)

Raven
 
Hi, Raven,

Good description. What the whole thing amounts to is that at some point, additional powder is wasted. It will not increase velocity and will usually burn outside the bore with a loud noise and a big muzzle flash.

Jim
 
Am I correct in assuming that at some point, the powder charge itself becomes more of the "projectile" and less of the "propellant"? I would imagine this phenomenon could be measured and graphed, with respect to bore diameter and bullet weight. Sadly, I lack the mathematical sophistication to pursue the matter further.
vanfunk

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semper ubi sub ubi

[This message has been edited by vanfunk (edited August 31, 2000).]
 
It has also been described in terms of the "expansion ratio". That is, the comparison of volume inside the cartridge case with the bullet in place to the volume of chamber and bore as the bullet just exits the muzzle. The longer the barrel, the greater the expansion ratio, and the less "overbore". It follows that the .30-30 is overbore in a 10" Contender, but not in an 18" carbine.
 
Ackley also went on to say that one of the additional symptoms of an overbore cartridge was the Barrel throat erosion that is often seen with the overbore ctgs. The 220 Swift comes to mind as a early notorious eater of Bbls.

Throat erosion can be attributed to the fact that some of the powder charge hasn't burned until it's actually in the barrel. The passage of these unburned grains is said to have a sandpaper effect on the metal. Ackley thought that the powder grains exiting the case and entering the throat actually wore the barrel and contributed to this erosion. Heat and pressure helped this phenomenon.

I guess the whole point of Ackley's arguement is that you will have a longer life in your barrel, will not need as much powder, and the rifle won't kick as much with a cartridge that isn't 'overbore' - in other words - it'll be more efficient. Hope this helps.
 
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