Whats an 'overbore bullet'?
The overbore refers to the chamber bore/rifle bore relationship. It is all about approach to the definition and It seams to me it is stated backwards, but one does not know the thought process of the one, many years ago, when coming up with the word. It is the standard way of stating the following information. I will discuss many things that lead up to the overbore rifle
In cartridge development, the manufactures have very sophisticated equipment now a days and can really get a better handle on what is actually happening. Dave Emery of Hornady stated that that the recoil from the acceleration of the bullet only represents 1/5 of the total recoil and the propulsion of the hot gases occurring, like a rocket, are still coming out of the barrel 4/5 of the time after the bullet leaves the barrel. Also there have been statements, various articles, that magnum cartridges use 20% more powder to gain 8% more velocity which is a general statement but not necessarily exact for any specific situation.
Now to get to the over bore statement, it refers to bottle neck cartridges that are in the Magnum range relative to non-magnum cartridges. The cartridge body is much bigger than the rifle bore(bullet diameter) and longer that represents much more powder relative to a non-magnum cartridge.
I am going to regress back to general geometry and air flow to show an extreme example to prove the point. Let us take a non-standard air tank and fill it up to the 55,000 PSI. The Air tank will be 12 inches in diameter and it will have a 1/16(.0625) diameter valve. Open the valve all the way and one could use a hand held stop watch to see how long it takes to discharge to atmospheric pressure. Now let us take the same tank, but put the a 12 inch diameter valve that will open the tank to the full inside diameter. The air will discharge quicker than one could use a hand held stop watch.
Now let us bring all the information together relating to an over bored cartridge. It takes a certain amount of time for all the expanding hot gases to discharge after the bullet leaves the barrel. The smaller the rifle barrel is to the cartridge diameter and the more the powder that represents the volume of hot gases the longer it takes to discharge the pressure down to atmospheric pressure. The hot gasses are as high as 3200 degrees Fahrenheit. The temperature spike is a very short time but it is substantially more, for a magnum cartridge, than a standard cartridge. The 3200 degrees is exposed to the barrel longer due to the time it takes for the hot gases to deplete to atmospheric pressure. The extend time of the heat will erode a magnum overbore barrel quicker than a standard non-magnum cartridge.
If one were to take the same cartridge and change the bullet to a larger diameter(re-barrel the firearm) to a straight wall cartridge, then the resistance to hot gas flow would be less so the hot gases would deplete down to atmospheric pressure quicker due to the larger bore. Then the barrel would not be exposed to the hot gases as long, thus the barrel would not erode as quick using the same amount of powder.
These are general statements that are generally true.
When living in the world of science and physics, many times when we gain one thing(velocity of a bullet) we lose something else(the barrel life). So we have to decide what is most important and take our best shot at determining the tools we use.
Best Regards
