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Someone please explain to me --
countershaft
Member Posts: 320 ✭✭✭
what is meant by " over- bore capacity"
Comments
Overbore
Etymology = English
over- + bore
Adjective = overbore
1. (firearms) relating to the situation where the capacity of the burning cartridge powder is more than the volume within the barrel.
Unfortunately, there isn't a simple explanation for the term `overbore'. The reason for this is the fact that there are too many variables involved in loading and shooting that can shift the balance from efficient to inefficient when a component or factor is changed. In essence, a cartridge that we would normally acknowledge to be fairly efficient can become inefficient and overbore when we modify some component or construction of the load. Note that I use the term inefficient as another expression for overbore. That's where the confusion comes in; people want to use the term `overbore' with regard to the cartridge and not the condition of the load as it relates to the barrel volume, which is the proper assessment.
Bear in mind that the goal here is to have a certain amount of powder burn within the length of a barrel to produce the gas or plasma, which pushes the bullet out of the barrel. An efficient load will, of necessity, burn completely in the barrel, long before the bullet exits the muzzle. This is simply because the super-heated plasma produced by the burning powder will continue to expand, filling the bore and acting to push on the base of the bullet after all the powder has been consumed. As an example; using a 22" barrel and a standard .308 Win. load, the powder should finish burning at 12" to 14" approximately. The plasma is still super-heated and expanding, filling the bore and pushing the bullet to exit the muzzle. If we add a greater amount of powder that requires all of the barrel length to consume the powder, the plasma will exit the muzzle right after the bullet in the form of a ball of expanding gas, which we all know as muzzle flash. The hotter the gas and the greater the volume of gas, the bigger the muzzle flash. All of that expanding plasma is wasted after it leaves the muzzle since it contribute nothing to the attained velocity. So an efficient load will have less muzzle flash because the plasma uses the last few inches of barrel to cool slightly while still pushing the bullet to peak velocity. Some Ball Powders are the exception, which when used to construct some loads, produce an excessive muzzle flash due to their formulation. These Ball Powders are coated with graphite for lubrication and a chemical referred to as a deterrent to control some of the excessive muzzle flash.
Using less powder in a given load doesn't guarantee efficiency either since the primer will push the smaller amount of powder ahead of the pressure wave created by the primer, leaving some of the powder unburned as it exits the barrel. A full case will restrain the powder against the shoulder and the base of the bullet allowing more of it to be ignited creating an efficient burn if the appropriate powder was chosen.
All of this is a means to explain that overbore isn't simply a big case with a large capacity. A common cartridge such as the .22-250 Rem. can be constructed in such a way as to be overbore if the wrong combination of components and the wrong construction of those components is used. A slightly slow powder with a light bullet will often result in some of the powder not burning (inefficiency). This can be demonstrated by placing a white sheet on the ground in front of the barrel and firing several shots. Lift the corners of the sheet and watch the powder collect in the middle. You will be surprised!
Almost everyone that attempts to explain overbore gets the big variables correct but they often leave out some of the minor details that can become very influential when we look at the whole picture. They want to quantify the big variables into some formulation that can be used to verify if a cartridge is overbore. The problem is that there are too many variables that can't be entered into a fairly simple formula for all of the situations that can occur when reloading. The best contributing factor for efficiency is pressure. Low pressure is inefficient and higher pressure is efficient. It is also the controlling factor for all reloading. Pressure must stay within the parameters that have been established for our safety. With this in mind, here is simple list of details that can have an effect on the efficiency of any particular cartridge.
1. Barrel volume (cu. in.)
2. Case capacity (cu. In.)
3. Powder Burn Rate
4. New Powder Formulation (better efficiency and burning)
5. Primer Choice
6. Length of the Bullet (longer bullets take up more space and increase pressure and efficiency)
7. Neck Tension (or crimp causes the bullet release to be retarded which increases pressure and efficiency)
8. Seating Depth (seating a bullet deeper in the case increases pressure, seating the bullet out decreases the pressure)
9. Longer barrels (allow some large capacity cases to become more efficient)
In a generalized reciprocal concept for most reloading, standard cases necked down require the use of slower powders to be efficient and reduce the tendency of being `overbore' Standard cases necked up usually require faster powders. What we're attempting to achieve is the efficient balance between the barrel capacity, the case capacity, the bullet weight/length and the velocity realized.
Best.